In my 98' SE 2.5L MTX, I get the same thing and all I have done to it is put SVT Exhaust on it. JSmith is right on the nose with this one... I think it's a charecteristic of the airflow of the exhaust gases as they exit the tailpipes... they are somehow getting in to the car a little... don't worry, it's normal... you smell it more probibly because you dug out your cats... do you not have any cat's now? Rotten egg is a cat smell.
