If You are getting an odor from only one vent in the house something has either fallen, or crawled in and is now decaying if You are comfortable with disassembling the duct work yourself then You could find this item and clean it out.
If not there are companys that will clean ducts for You.
If You need more info feel free to ask
Sorry for the delay I was having technical trouble.
If the smell is being pulled in to the system it would disperse evenly from all vents.
Just a wild guess here but the A/C uses High speed fan as where the heat would only use low speed if some how the vent pipe from Your sewage ran through the duct work( I have seen this before along with other things, cable, gas, and water lines) You could get enough pull to bring sewer vapors into the duct other wise its under pressure and would tend to blow out through any leaks in the ducts so it would not be likely a smell could be pulled into a single duct.
this is about the only thing I can think of other than something in the duct such as an animal, but then again that gets questioned since the smell only occurs when the A/C is used odors from items would tend to be more strong with heat.
I am going to have to say that there is likely a pipe that has been shortcut through the vent and the high speed fan is able to pull a vacum on this pipe and pull in odors.
Let Me know what You find