Hey guys. Just got my new header-back exhaust in. I was running on open headers for the weekend waiting for the exhaust shop to open on monday and it sounded like crap. But i stuck my 60 series flowmaster i got up on the header pipe, and it sounded pretty good. I even revved it up and it sounded pretty good. So the next day i took it to the exhaust shop and got some 2.25" exhaust all the way back, and he welded the muffler on for me. Now here's my problem, it sounded fine when i had it on my header pipe, now when i get up to about 3000 rpms it starts this horrible rattling type sound from the muffler. You can kick the muffler and kind of hear something inside rattle, but not really. I'm not running a cat or anything, and no resonator. Any help at all will be appreciated. BTW this is on a 1996 2.2 Sunfire. Thanks in advance.

Check your muffler bearings

^^^

But to be serious, flowmasters tend to rattle one the metal baffling inside begins to break down and fall apart. But I don't see that happening if its a new muffler, so you may have gotten a defective piece.

Alright well i got ahold of summit and they need me to cut it off and send it back, then they'll send me another one. So if this new one doesnt fix the problem what next? its not hitting anything under the car that would make it rattle. and all the pipes are welded so they arent rattling. I've heard of straight pipes into a muffler being kind of "raspy" but never clangy or a metal on metal type sound.

It also could be caused by you not running a cat or resonator. Flowmasters tend to have a metallic sound to them until carbon starts to build up.