I just had a new home built and I'am having a problem with a bad smell coming from my vents. It doesn't happen every time the system is on. It seems to happen when the system is working hard. The smell is like very old, sour, wet wood, or a very dirty bathroom. Does any one know what this is? If this is of any help, it is a heat pump system and I have two in my home. It is only happening with the unit that supplies the heat to the first floor.