I have a question for you all. I swapped the american thunder flowmaster catback that I had on my 99 SS to my 94 Z28. I had a custom 2.5 ory made for it at the same time with the flowmaster y collector. My problem is there is a rattle in the car only at certain rpms under moderate accleration. Usually happens around 1800-2500 rpms. Is this just a characteristic of the 80 seires muffler on a LT1 car? It would sound awesome if it wasn't for that. The car is an automatic by the way. Any suggestions on what it could be? I have removed the sheet metal heat insulator on the passenger side of the car where the cat was. Would I be better off to go with a single 3" 40 series flowmaster in place of this setup and have like a custom 1 or 2 out on the left setup?Here are the best pics I have of the setup as of now.

 

I have the 3" and had some rattling at first. After about 2 weeks or so of driving it finallay quit doing it. I don't know if that answers your question but you might want to just drive it for a while and see what happens.

 

I am having the same problem. I have the GMMG and put it on probably 3 months ago. No rattle whatsoever, now for some reason here lately I have had a really bad rattle that sounds like it is on the passenger side. It only does it when im decelerating. What could it be???