ok when i go to take off from a stop I get this rattling or vibration, I just put the gmpp catback on. and sometime when i go from 2nd to third i feel it and hear it a little. so any ideas?

maybe the heat shield vibrating or the downpipe, i don't know help me out

 

Or it could be the tip bouncing? I don't know...

 

i put my GMPP exhaust on and was fine for couple hundred miles. after bout 300 miles when i first start the car or shut my car door i can hear the rattle. i went underneath and tightened the exhasut up real good. nothing since than

 

Maybe your tip - get that bitch welded on. Where's the rattling coming from?
If it was your connection to your cat, I'd think you would have some level of exhaust leak.. so I'd say heat shield or tip.

 

x2, same thing happened to me. Its always good after a install and a few miles to re torq everything to the proper torq specs. Adjust anything if needed. My exhaust moves a little bit too and I had to adjust the tail pipe.

 

now you know on the stock down pipe how it has those ring looking things, is that flexable at all cus when i took the stock exhaust off i had to move it a little. i will take it to a hobby shop and see what i can find i know everything is tight and its not the tip the noise is coming from like the rear of the engine to about under the seats

 

I had a similar problem, mine was a rattling / buzzing noise when starting off, and shifting from 1st to 2nd, after the exhaust warmed up the noise went away. It was my resonator, I can't remember what it was specifically what was wrong with it, but it needed to be replaced.. They ended up replacing my exhaust pipe and the resonator back in July. That fixed the problem, untill a few weeks ago, I've slowly started noticing the rattling / buzzing again.