I am having a problem with diagnosing a very distinct smell from one of the AC vent located in the living room of a home. When the AC is turned on, after 5 minutes, a smell like from the horse staple comes through only one (1) of the AC vent in the house. I hired a AC guy to come out to check on the AC, can't find anything wrong with AC. This vent sits on top of the sewer line. I had a plumber go into the crawl space but cannot find a leak either. The clean out is right outside this vent. The other vent in house doesn't smell. Does anyone have a clue as to what's going on?