I am smelling sewage gas from my ac vents... mostly the return vent which is at the ceiling in a small hallway... the smell concentrates in that hallway.. there is a half bath in that hall as well... the smell is not so much in the bathroom... i also will get the smell on the other side of the house near a bedroom .. also near a bathroom... i thought the p trap with the ac was dry...however i bugged by builder and he cut the pipe and it is full... the house is only 6 months old.. i dont know if this is related but off that same hallway with is my laudry room and when i run water into my washing machine,,,i get a horrible rotten egg smell that goes away after the water runs for a bit... could it be related
i do not get that smell from any other water source but that one.
we have city water...

If you are on a well the rotten egg smell is from the well, it may be a sulfur smell, I am not sure what you can do for that, maybe a well man will answer.

I would think it is a dried up trap, poor ventilation, or even the plumbing vent is loose in the attic or something along those lines.