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?

Maybe too simple an answer. The vent is drawing the odours from the sewer line due to reduction of air pressure outside. How about trying to cover the sewer vent with a tied-on piece of plastic. completely shut off air around the sewer vent... for a time while you run the AC unit.
See if any change is noticed.

If it fixes such odour, then you have found your problem source. If not, look for other cause.

You might consider getting the pipe system cleaned out and see if possiblly something is caught in the pipe and the air passing over it picks up the odours.
Many times, such odours occur in heating systems because dust and other particles over time get caught up in the pipes. Kids throw food and other articles down the vents... and there it sits until it rots.

How long have you had this problem. The problem will not be that something is being sucked in to the ductwork. The A/C fan is blowing air into the duct creating a high pressure. Any leak will cause air to go out of the duct not come in, just like it does at the vent. Most likely candidate, mold or mildew inside duct. Does the duct have insulation on the inside?

HI Mingchu,

Did you ever find what the odor was or how it was coming from there. I have the same problem in my house. Bad odor coming from 1 vent in the living room.