We have a two year old house that has a bad smell that seems to come from a particular A/C duct. I grew up in an old house and occasionally a mouse would die in the walls and decompose - this smell reminds me of that, but not as strong. Here is the strange part, the smell will be in the room for a couple of hours at a time, then go away for several hours and then be back. There is no pattern to when the smell will be there or how long it will last. We first started smelling it about a 6 months ago when we smelled it almost every evening for a couple weeks. Then for about 3 months we didn't smell it at all and now its back. I've crawled around in the attic looking at the duct for any loose joints or condensation and can't find any. I've also looked into the duct from the vent and don't see anything. I also haven't seen any dead mice or bugs in the attic that would cause the smell. Now I'm thinking maybe its mold?!? Does anybody have any experience or suggestions?