When you upgraded your cat-back exhaust, what gasket did you use in between the downpipe and cat-back? The Vibrant header/downpipe combo doesn't come with the gasket and neither does the Magnaflow cat-back exhaust kit.

 

the one that fit
measure how big the hole is and how far the bolt holes are offset then go down to your local parts store and give it to them, in mine they just tell me to go behind the counter and find one.