I got this rattling/buzzing noise that is coming from
the back. Car will only make the noise when accelerating,
in any gear. It starts around 2300 RPM and seems to go away
around 3500 RPM. It is pretty loud.
It started shortly after installing Pacesetter long tube headers
and Y-pipe. I'm emmisions exempt, but I reused my Magnaflow
cat and put it in the passenger side of the Y-pipe (room).
So far I eliminated: Knock (100 octane gas), exhaust leaks
(rubber hose , seafoam), exhaust is not touching anything,
heat shields are good. Catback is almost new.
The car will not make the noise in park, and it seems to be
coming out of tail pipe, the best I can tell when driving.

 

It could be that cat back, even though it is new doesn't mean it won't knock on things during acceleration, or while driving. I had magnaflow and currently have SLP and they tap a little under accel.

 

Just a guess, but perhaps the header install leaned out your mixture, killing your cat, cracking the substrate inside which is now rattling. If the noise is coming out of the tailpipe as you think, there's nothing else I could think of to explain that. Can you remove the cat to check? You're exempt anyways, why keep it on there? Good luck

 

I tried hitting the cat with a rubber hammer, and id doesn't rattle.
It's welded in.
Is there anything in the engine that would make noise, that can
be heard out of the tail pipe, but not from the engine.
Valve rattle maybe?