What did the Underground Railroad smell like?

Why is there a Fishy Smell Coming from Mudroom below Central Air Conditioning Unit in Attic?

• All last summer a strong fish smell seemed to be coming from our mud room (no a/c vents in there, but there is a trap door in the ceiling to the attic which is right next to the a/c unit).Adjacent to the mudroom is our den which has 2 a/c vents.The smell is pretty strong in there too, although it seems stronger in the mudroom. We never figured out what the smell was & hoped it would just go away. The smell only came when we turned the a/c on & no other rooms (aside from the kitchen which is next to the den)got the smell. Yesterday we turned on the a/c for the first time of the season & it had the same horrible fish smell from last year.My husband went up into the attic, there was no smell by the a/c unit, the condensate pan did not smell either. Years ago we had a problem with the pan not emptying well enough due to the lack of incline & it would leak over the side through the garage ceiling.But it was corrected we think & has not leaked through the ceiling since.We were thinking yesterday that maybe an animal got into the duct work to the den last summer and died (doesn't explain the mud room smelling though) so for lack of a better idea, my husband went to Home Depot & bought new ducts, which he replaced yesterday.BUT, after the new ducts were in, we turned on the a/c & the smell was just as strong. Today we taped up all the a/c vents in the den & sealed the door from the den to the mudroom with a plastic drop cloth & tape. We turned the fan on for quite awhile & no smell came into the den. & I don't think the mudroom smelled either, although there was some odor lingering from yesterday (the mudroom has no vents but a trap door to the attic where the a/c unit sits.)We tried our best to seal this trap door too, but I don't think it was air tight. We then turned on the a/c, no smell in the den, but I think the mudroom became smellier.We took the tape off the vents in the den & no smell came in while the a/c was on (the den door to the mudroom was still sealed). So that's where it stands now.Smell is definitely not coming out of the vents/ducts to the den.Definitely starting in the mudroom where there are no a/c vents, but only starts to smell when the a/c is on.We are at a loss.Only thing we can still think of is that an animal died in a mudroom wall.But why would it only smell when the a/c comes on?Rip out sheetrock next to search for a dead animal? But from what I've read a dead animal smell should only last a week or two. Any suggestions would be very appreciated. Thanks in advance:)

• Answer:

  possibly- something in the ducts, filters, condensation making something wet and smell