When the fan turns on in your compressor unit, it should not rattle. These sounds may be an indication of loose hardware or a failing motor. The louder the sound, the more serious the problem can be.
If the noise is similar to a scream, your compressor may be failing. Humming noises can indicate the starting capacitor is starting to fail or a bad motor. A squeal when the unit first starts is often normal. If the sound continues, hire a heating and air conditioning professional to check it out.
Have you had an experience with your HVAC system making strange noises? Tell us about it in the comments section below.
Editor's note: This is an updated version of an article originally published on Feb. 5, 2013.