A common problem encountered during the cooling season is so-called dirty socks syndrome. A foul odor produced by a heat pump, it is most often caused by an accumulation of biological contaminants on the indoor heat pump coil.
According to Manual D, Residential Duct Systems, Air Conditioning Contractors of America, "Researchers speculate that microorganisms - which are commonly found in the soil, water and air - are deposited on the indoor coil during the summer, when the coil is cold and wet, and that these colonies thrive and grow during the heating season, when the indoor coil is dry and warm. However, this coating, by itself, does not produce the odor problem, even when spores are entrained in the supply air. Evidently, the odor is produced when the airborne contaminants are burned. This condition can only occur during the heat pump defrost cycle when the electric resistance heater is energized, but even then, it might not occur unless the indoor coil is wet."