We are in our second winter in our new home, zero problems last year. As winter approached this year, we decided to make use of our lovely woodburning stove for heat... all good. With the recent frigid air, we decided to kick on our furnance as well. It is only four years old, all new ducts, and is powered by propane. Our home is a one level ranch, 20 years old. This 'rancid' vinegar like odor is coming from the heat ducts (which are in the ceilings) in our master bedroom and our daughter bedroom, both rooms are farthest from the furnance. It is powerful, so bad that my daughter cannot sleep there. We have closed both vents in our rooms, but there is still a bad odor present. Just today, I slithered on my hands and knees on the skinny 2 x 4 boards in the attic crawlspace, sifting through insulation, trying to detect any odor causing source. All duct work appeared intact. It does not smell like a dead critter, it is more of a rancid vinegar smell, I am concerned about mold. Also in our guest room, there was a condensation stain around that air duct that we noticed a few months ago. we do have central air, and rarely used it this past summer. We live in michigan. Please help, my husband does not know what to do and is not as concerned with this as I am.