Check for mold buildup on your furnace coil pan
Having mold in a furnace coil pan (this is where condensation is collected) is not uncommon, due to the high volume moisture in the device. Using a high quality air filter can help keep pollutants and particles out of your HVAC system. Humidity in your home may also play a factor in terms of mold. UV lighting could help alleviate the problem. You may, instead, have the area sprayed with a micro biocide sanitizer that can help control harmful organisms.
It is wise to have your ducts inspected to determine if mold growth is widespread before you get the entire system cleaned. Many companies offer video inspections so you can see for yourself what is in your ducts. Beware of the companies that offer the $49-$99 cleanings. A quality air duct cleaning typically will cost several hundred dollars and it will include all components of your HVAC system, including the coils, drain pans, fans and humidifiers. If you have a problem and it's not fully eradicated, it will just come back. If you have widespread mold, you should hire a reputable company that specializes in mold remediation. Always get at least three estimates from certified and trained HVAC technicians before having any work done.