I have an unusual, old milk smell coming in through my heating vents at times. If the heat comes on 10 times per day, I would say the smell comes with it 2 times per day or less. We had been using the heat this season for at least 1 month before this started happening. We have a heat pump and the smell comes through in every room. We changed the filters and did a sniff test everywhere, including in the basement and can't find the source. Anyone have any ideas?