I have a 2 year old Tappan 13-seer a/c-heat pump that emits a musty odor in the house for a few minutes every hour or so on colder days when the heat pump is set to heat and it is in defrost mode. I know it is in defrost because the compressor fan outside stops running, while the compressor itself is on and the fins get hot. This musty odor is never present when the system is in emergency heat mode, so I don't think the problem is in the inside heat strips. It is also never present in AC mode.

You have what is called the dirty sock syndrome. Mold,mildew, and other bacteria. The coil needs to be cleaned.