We have a bad smell as well. We do not know what it is, but think it is in our ducts. We had our ducts vacummed out, but it did not help. Later that night our 1 year old had breathing difficulties and we had to take him to the ER. He has had no problems since so it may have been that dust had gotten stirred up. Our home is on a slab. I don't know if this has anything to do with it, but some of our duct work has come loose and you can see parts of the slab. We do not know if the slab could have mold or mildew in it causing the odor or if condensation is building up in our ducts and mixing with dust causing the odor. We will be calling a building inspector. If you have any ideas please let us know.