I'm living in a new house less than 2 years old. The heating system is electric with a heat pump. Some times when the heat is on, a sour or musty smell comes from the air vents. The filters are replaced regularly. The smell only occurs occasionally, usually when we've had a lot of rain. Any clues as to what could be causing it and how to get it to go away? Thanks.

If the unit is in the basement it might be picking up a drain odor from there.