I'll say here that I haven't read all the posts - but I had a musty smell in my place and finally checked with a plumber who told me that if I had a drain that wasn't used a lot - shower, tub, etc, the musty/mildewy smell could easily be coming from there - my cure was to periodically run water down the drain and keep it "hydrated" and the musty smell stopped. Water in a drain at the "J", or is that "P", trap keeps smells from coming back into the house...if the drain isn't used, the water in the trap dries out and smells from the drainage system can come back into the house...