What's wrong with my 1998 Cobra??!!?

I have a 1998 Cobra, and last week I took it to a local muffler shop to get an after market cat-back exhaust system installed, after the guys were done with it, they started it and it sounded awesome! but on my way back home I noticed an air leak coming out of the penger side that wasn't there before!!, I've tried to pinpoint the exact location of the problem with no luck and believe me IT'S ANNOYING!!, I don't want to take it back to the shop without having an idea of what the problem might be, I'm only 18 years old and I don't want to be taken advantage of or being ripped off so any advice would be most appreciatte it!!