i think that farting noise is just the cat. changing the way the exhaust sounds. my friend got a 2.5" header, 2.5" cat, and 2.5" exhaust piping all the way through it sounds really metallic, and similar to what you are describing.


I think it's just the cat, plus the diameter change within' the cat would be doing it also.

wouldn't worry about it, unless it's a scraping sounds or the exhaust starts hitting something.