Exhaust Sputtering

Ugh! Another little problem!! I went through the car wash the other day (brushless), which has a strong underbody spray. 1st problem (unrelated) it cracked and blew off one of my Ventshades. Real problem: I think it poked a hole somewhere in my exhaust. I know there was some rust underneath the car, but never actually inspected the exaust for rust. Anyways, I noticed it the other day after my 60 mile commute. When I was sitting in the driveway I heard a sputtering, almost like air puffing through a hole in the exhaust. Now, it does it all the time, though not noticeably from inside the car - only outside and typically at rpms less than 1500 and around 3500 it rattles. It seems to be coming from near the Cat. What is the warranty on the "emissions system," which would include the cat? I now have 71,000 miles on it and have suspected cat problems in the past 10k miles, but the dealer said they did a backpressure test and it was fine. So maybe a pipe? U bend? Not sure. No SES light yet. Defenitely from exhaust in that area - below car, behind engine, yet in front of rear seat. Any suggestions before I take it in? If the light comes on I'll get to autozone (or use Onstar if I ever get it fixed - see "Aux Input Static" post). Anybody have this before that can tell me what their problem was?

Cat is 8/80 or 7/70 depending on Calif or not, check your warranty booklet.. Get it on a lift and see what's what, no other way here.

I don't have my warranty booklet, but I live in PA, so I'd assume it'd be the 7/70. If it is indeed the cat, it better be covered, cause I'm only 960 miles out of warranty and had it checked back in August cause the exhaust was making funny random clicking noises when warming up.