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This can be maddening. Over the past 40+ years, in 4 houses, I have had or have run across this problem from gas meter leakage, water well pump column vibration, doorbell transformer, circulating pump, an extremely small (mist spray) water pipe leak, flourescent and sodium lights, security system horn dead battery, gas meter leaking slightly, bees in wall, bat colony, electric typewriter left on, stereo left on very low, and speaker inductive hum.

This seems to be a popular and recurrent question, so I am going to give the long answer for use by future questioners too.

I am assuming you do not hear this noise away from your house, or that other family members can hear it to. Obviously, if you hear it elsewhere also and other family members cannot hear it, then maybe you have tinninitus or are hearing your own high blood pressure blood flow (seriously). This commonly gets more acute at night when it is quiet, so all you are hearing is your internal ear sounds. I had this happen once because of a middle ear blockage - drove me crazy, getting up in the middle of the night because I thought I heard a water leak through the walls. Try putting on a pair of earmuffs or hearing protectors - if you still hear it or hear it louder, this is probably the case.

One method if hum is on the clearly audible side is make a 2 foot long cone out of paper to hold against your ear - like an antique hearing horn - then in each room face each of 4 directions while listening for where sound is the loudest, and turn your head to pinpoint the exact direction - I would spend 10 minutes doing this before getting into detailed stethoscope listening.

Otherwise, sounds like time for the old stethoscope (about $12 at a drug store - get a metal soundhead one, not cheap plastic, which does not pick up vibration as well). Also, if you are older (say over 35 or so) your hearing might have started to deteriorated with age, so if you have children or grandchildren with sharp hearing, they might be able to help track it down. I am sure a young child or grandchild, if you have one, would love this sort of treasure hunt (with appropriate "treasure" for a reward for tracking it down). 

Being careful not to come in contact with electricity with the stethoscope, check all the likely sources listed below. Start by placing it against pipes and walls and floor in each room of the house - water sourced noise goes a long ways, and tends to reverberate in the walls, so if that is the source likely to hear pretty easy. Hold stethoscope against bare pipes, both hot and cold, and heating system radiators or hot air vents.

If listening to water and hot water heating pipes indicates it is not water sourced, then you could turn off the master (outside) breaker or all the inside breakers and see if it goes away. I would only do this during above-freezing weather and early on a weekday, just in case a breaker fails to turn back on correctly when you switch it. Older master breakers particularly, which typically have never been used, sometimes break or fail to reclose properly after being shut off, so then have to be replaced. You want to be doing this at a time of day when, if necessary, you could get an electrician in the same day to replace it without paying weekend or nighttime emergency call rates.

If turning off the master breaker (or all other breakers) eliminates the hum, then turn them on one at a time until you find the one that turns the hum back on, then track where that circuit likely feeds (hopefully it is labelled) and check every switch, outlet, and light fixture.

Humming sources include (not in any particular order, a + in front means likely or common source of humming, - means rare or not likely):

1) + toilet fill valve - slightly leaking toilet inlet valve (listen where water tubing comes into toilet tank, and look inside tank to see if there is any water flow into or ripppling of the water in the tank or the bowl, or from the bowl filling tube (usually a small black plastic flexible tube which comes out of the fill valve (usually far left side of tank) and is clipped onto and discharges down into a hollow vertical brass or plastic tube or pipe in the toilet tank, which refills the toilet bowl after you flush)

2) + leaking faucet - kitchen, tub, shower, sink, utility tub, etc - it is amazing how just the smallest valve leak can make a hum or hiss that you can hear through the walls (especially at night), but only drips every few seconds.

3) - electric service meter dial motor

4) - electric breaker panel - rarely, a loose main power feed to a panel (especially with aluminum main service wire) will get loose enough that it vibrates back and forth and hums in its connector. A loose bus or snap-in breaker slot cover plate in the panel can also do this rarely

5) - gas meter or overpressure vent (unlikely, as you have had it replaced)

6) + boiling in the bottom of hot water heater or boiler because of buildup of lime, but would usually be intermittent - only when unit is heating

7) + furnace fan or electrostatic filter (forced air heat), or circulating pump (hot water baseboard heating), or steam condensate pump or overpressure venting (steam system).

8) - gas control valve or electric control box on a gas furnace, or its transformer (most have a 120V to 24, 16 or 12V transformer inside the front of the furnace

9) + air filter or electrostatic filter alarm on forced air furnace - some have a passive "whistle" opening that sounds softly when the filter is getting blocked, and if blocked with dust could make a hum rather than a whistle.

10) + Some water softener systems also have an "alarm" device to tell you it is time to service the unit, so check that if you have such a unit.

11) - a slightly leaking overpressure/overtemp valve on hot water heater or furnace (would be dripping)

12) - air venting from the air vents on hot water heating system. These will commonly make a hum or wheeze sound, for only for a few seconds at a time - not continuous unless leaking water

13) - city water system booster pump sound through the water column (if there is one near your home) - listen at the incoming water pipe - if much louder there than at other pipes within the house, that could be a house, though unlikely. If you think this could be it, find your water shutoff valve (typically 10' into your lawn from the street) and listen there. Would also be audible at neighbor's service pipe if that is the source.

14) - gas system compressor sound coming through gas pipe - listen to gas pipe outside the house and inside the house near furnace - if louder outside,, this could be a possible source, but the compressor or pressure reducer would have to be near your house. Would also be audible at neighbor's service pipe if that is the source.

15) + auxiliary booster circulating pump in your hot water or steam heating system (there may be one separate from the furnace, likely in the basement or a utility closet - most commonly found on  multi-unit apartment building with central heating and in 3 story or higher buildings, but you never know)

16) + a water leak, either inside or a leaking hose bib or pipe, or in your service pipe coming to the house

17) - electric on-demand water heater or electric-powered water filtration unit under the kitchen sink or inthe basement

18) + refrigerator compressor or fan hum

19) + doorbell transformer (front or back door - transformer is usually NOT at the doorbell, it is usually mounted in an open space like nailed to a basement joist, in an entry closet, or in the cubby space under the stairs - always physically near to the door, but NOT always on the same floor)

20) - any instant-on device like a TV

21) + any audio device (stereo, iPod, music player dock, computer, etc) that may have been left on at very low volume. Also, VERY rarely, if stereo or external speaker wires are run close to and parallel with an electric wire in the wall, they will acquire an  inductive voltage and hum.

22) + anything with a transformer, including stereo, add-on computer or iPod speakers, battery charger (rechargeable batteries or spare car battery or rider mower or boat battery charger), any portable electriconic device. Also portable device chargers (computer, iPod, cell phone, etc) - even if the device is not plugged into the transformer, as long as the transformer (charger) if plugged into an outlet, it is transforming high to low voltage, and transformers commonly hum

23) - electric typewriter left running

24) - electric ultrasonic cleaner or denture cleaner or electric toothbrush left on 

25) - home hair drying hood left on

26) - a lint buildup-jammed bathroom, kitchen, or attic fan. Many of these have, for safety, so called "self limiting" motors that if they jam just sit there and hum, but do not burn out.

27) - an attic cooling fan whose thermostat has failed, so is on all the time

28) - electronic furnace thermostat

29) + air conditioning unit, or aquxiliary air conditioner evaporator

30) + humidifier / dehumidifier - either permanently installed or portable

31) + portable heater / fan / air purifier

32) - automatic animal feeder waterer - either water supply or electric, as applicable

33) - dishwasher motor runningcontinuously - not shutting down after end of cycle

34) - convective or direct-vent oven or cooktop exhaust fan not shutting off

35) + flourescent (tube or CFL) or sodium or halogen light bulb / ballast hum (either inside, outside front door fixtures, or public street lights). These can hum quite pesistently when the starter circuit sticks on, or the bulb is dying and will not start (light completely), so the started circuit tries continually to start the lamp - can make a hum audible up to a block away on street lights.

36) - a dying electronic photocell designed to turn on your outside lights

37) - home security system, especially its alarm or horn. If the alarm is sounding but for some reason the main power is not getting to it, then as the battery goes dead (or if full voltage is not getting to it) is can give off a squeek, hum, or rasping sound - ditto if insects like wasps or hornets build a nest in it, so it cannot sound correctly.

38) + well pump, pressure tank, or filtration system, if you are on a well

39) + insect or bat nest in the attic or walls or in outside bins or cupboards, electric panel/meter, or outside telephone connection box (bees /wasps / hornets most likely) - though this usually varies by time of day, although it would "pulse" at the time of day when they are waking up or going to sleep.

40) + carpenter ants or termites - their continuous chewing of the wood can sound like a hum till you get right up against the colony, then you can actually hear the chewing

41) - a regional hum, as has been occurring at times in Ohio, Wisconsin, and Arkansas - where micro-seismic activity causes a hum or booming sound. Google or call your local paper and see if anyone has been reporting this in your area.

42) + outdoor power service transformer - either a metal (typically army green or gray) about 1 foot diameter "can" mounted on a power pole if you have overhead service, or a 2-3 foot cubic metal box on the ground or in a manhole pit near the street if you have underground service, which usually serves 4-6 houses (so may be in a neighbor's yard) and will have a voltage rating marked on it, usually in yellow stick-on lettering - like 4160V - 220V. Usually has high voltage - keep away safety markings on it.

43) - you have found where the Caddyshack gopher (who hummed to himself) moved to after Bill Murray blew up his happy home at the golf course.

Hope this list helps you (and future users with the same question).

Click on the Home > Electrical link right below your question - you will find a number of questions like yours with answers about panel and service upgrades, and factors which affect cost.

I would get a couple of opinions from electricians on the general panel upgrade issue, unless you want to do that anyway for general upgrade purposes. If you are upgrading the main panel from 100 to 200A, then yes you have to upgrade to AFCI and GFCI breakers (as applicable) as part of the process. 

However, if all you want to do it install power for the electric HVAC system, then it may be a lot cheaper to just upgrade the outside service capacity if necessary, and then install a dedicated secondary dedicated panel for HVAC system use, without touching the existing main breaker panel. Could make a 2 or 3:1 difference in cost, depending on your current situation.

An electrician can help you calculate your needs.  If you want everything to run as normal while on the generator you will need a fairly large setup.  Add the amperages of every device and appliance you expect to use while on the generator including the refrigerator, freezer, HVAC (all components), TV and accessories, lights, etc.  Wattage (which is how generators are measured in terms of output) is calculated as Volts x Amps = Watts.  So a 120v appliance using 15 amps requires 1800 watts.  A 240v appliance using 30 amps (like an electric water heater) requires 7200 watts.  Don't forget to calculate starting amps, not just running amps.  Appliances with motors and compressors require more power at startup than they do once running.

A simpler way to cheat/estimate your need is to count up the breakers in the main panel of your home.  All single pole breakers are multiplied by 120 while the double pole breakers are counted together and multiplied by 240.  While this method will give you a rough idea of what you need it may not account for all of your useage.  Most people don't hook up standby generators to the entire house.  Instead, they have an electrician selectivly wire which circuits are powered on the standby switch.  Then when the power goes out and the generator comes on only the essential devices are running.  This saves money on the generator as a smaller one can be used as well as on the fuel used to run a smaller generator versus a larger setup.

Most typical homes have a 100, 200, or even 3-400 amp service panels and meters.  If you want to run everything consider a generator that matches your existing service from the power company.  A 200 amp, 220v service is 44,000 watts.  That is your maximum available draw but it doesn't mean you really use that much.  Most people don't.

If you already have a fixture there, around $100 plus or minus about $30 if not over 7 or 30 pounds, or about $200-300 if weighs over about 7 or 30 pounds. The 7 or 30 pound ranges are dependent on whether the existing box is plastic or a standard Square-D type or equivalent metal one, because each type of box can only hold so much weight. If existing box is rated 70 pounds or more (ceiling fan or large chandelier rated) then the $100 range should do it regardless.

I am assuming when you say mini-pendants you meant from a single box if more than one. If you meant two separate by some feet, add probably about $100 more to add a second box, ASSUMING they will both be run off the same single switch.

IF you do NOT have a light fixture there now, then can run from $150-300 range if there is easy access from above (open attic) to $500 or more if has to tear into drywall to install wiring and box so you then have to get drywall contractor and painter in to repair the damages.