a/c smells like exhaust

Ok here's the problem... when I stop at a light my a/c smells like exhaust. When I start driving again the problem goes away and it smells fine. I've been told that my exhaust system is fairly rusty (from being up north), but, It'd have to be real broken to produce the kind of exhuast it would take to fill up the engine compartment and spill into the vents. The problem is that it isn't loud at all.

Any ideas as to what might be causing this?

When that happened to me, it was my cat converter. About $180 for a brnad new one.

Yeah I had a bad one too, but, I just replaced it and got a new one and it didn't help.