We hear this complaint mostly from new heat pump owners at the beginning of each heating season. Sometimes they think it is smoke and that their outdoor fan motor burned-up because when this is happening the motor actually stops running.
Don't worry, this is a good thing! This is your unit during the "Defrost Mode". It happens regularly during the heating season. The outdoor coils tend to frost or ice-up during the winter. This causes the unit to loose efficiency. By regularly defrosting itself, the heat pump runs more efficiently.
When the unit goes into defrost, a couple of things happen. First, the outdoor fan motor stops running. This helps build-up more heat to melt the ice. Also the reversing valve shifts from the heating mode to the air conditioning mode. That's right, you are actually running the air conditioner. By making the outdoor unit the condenser, the hot freon gas passing through the coils accomplishes the defrosting. Lastly, the supplemental heat is energized to offset the now, cold air blowing in the house.
Yes, it does sound a little bizarre to run the air conditioning and back-up heat at the same time during the winter. But it usually only happens for a few minutes at a time and only when needed.
Now, this should only happen periodically except for severe weather conditions (snow, rain, sleet). If your unit is constantly going into defrost, this indicates a problem.
Below is a list of possible causes.