I'm hoping that someone on here can help me out with a mystery smell coming from only one room's diffuser on a heat pump system. It's a 2 1/2 ton system that has been in use for about 18 months. The filters have been changed on a regular basis (30-45 days). The smell only occurs off and on and has been doing so for several months. It doesn't get any worse, it just will go away all of a sudden. I've checked the insulation at the connection point to the diffuser to make sure it wasn't a condensation mold issue and there didn't seem to be a problem. I've looked around the attic space for dead critters and wasn't able to find anything. I'm baffled. If it was something dead, moldy or otherwise, you would think that the smell would get worse or go away, not come and go for several months. Any help would be appreciated.