I wrote this response awhile back, can it be dirty socks syndrome?

From information gathered over many years, most professionals agree the "Dirty Sock Syndrome" is caused by a bacteria that collects and grows on the indoor coils of heat pumps and air conditioners.

Most customers describe the odor as a musky, dirty, locker room smell. The problem is similar to an odor that is blown out of a car air conditioner when the AC has not been used for a while.

It is very important to properly identify the problem before any action can be taken, since many odor problems are incorrectly labeled as a dirty sock problem. Eliminate dirty drain pans holding water, drain lines connected to plumbing systems without adequate traps or dry traps, return air leaks in ductwork or chases, or dead animals in ductwork or near the living space. If the odor is present ALL the time, especially during heat, the problem is NOT a dirty sock syndrome complaint. Dirty sock complaints only smell when the indoor coil gets cool and the bacteria releases its odor into the air stream.

The Dirty Sock Syndrome plagues 0.5 to 2 percent of heat pumps in the southern states, with Texas being on the lower side of the percentage. The syndrome is not brand specific, with all manufacturers acknowledging complaints. The problem itself is sporadic and limited to isolated households and is somehow related to the living style or products in the home. This can be proven as Trane and other manufacturers have documented changing out systems with new product and the complaint returns. After removing a "stinky" unit from a complaint house, the unit can be cleaned and installed elsewhere without a complaint surfacing. Changing the brand of equipment is met with a similar lack of success. Much effort and expense has been given by the industry to research and solve this syndrome.

Once the problem has been properly identified, action can then be taken to resolve or reduce the complaint, starting with a thorough cleaning of the evaporator coil with a non-acid coil cleaner. Cleaning will bring the system back to normal and will usually prevent a complaint for the rest of the heating season. Some systems cleaned early in the season or those having more of a problem may have repeated problems during the same season, especially if the weather conditions force a system back and forth from heating to cooling. The majority of complaints are resolved with a thorough cleaning.

If the coil cleaning does not resolve the complaint to a satisfactory level, the next suggested action would be to clean the coil again and apply a coating of Alathene II, a special spray designed to continuously protect coils from fouling caused by airborne contaminates. In addition, a UV light system at the coil may retard the bacteria from comming back. If seasonal cleanings or the application of Alathene II does not resolve the complaint, as a last resort, your coil may need to be replaced.