During the winter months, one of the most common calls our office receives is that our customers noticed their outdoor unit making strange noises, or they saw smoke coming from the outdoor unit. There is always the possibility that there is indeed a valid issue that needs to be addressed, and if they are indeed seeing smoke, they may need the fire department more than an air conditioning company. So we ask a few more qualifying questions and help them come to the most likely conclusion. Their heating system is a heat pump, and it has gone into “defrost”.