I posted this in the intake/exhaust board but got very little response so i'm reposting it here. I hope the mods don't mind:

I have a problem with my Z and I need your help finding out what it is...please. I think the problem is somewhere in my exhaust system but to give you a bit of a background on it, I have an 03' Track w/ Nismo intake, headers, exhaust, and Crawford cats. Anyhow, whenever I decelerate and sometimes even accelerate through about 2,250 rpm and again at 3k rpm I hear a 'rattle' coming from the bottom of the car. I thought it was a loose bolt or something similar but I just got back from a shop close by and they said everything was 'tight' and that my problem was internal and most likely the 'resonator'. Does this sound accurate or does it sound like I have a leak? It sounds like a piece of metal vibrating and it is very annoying. I don't think i'm losing power but i'd like to get it fixed asap. Any ideas on what it is and what I can do? Thanks for listening...

 

Have you checked the clearance between the crawford cats, and the underbody heat shield? How about the exhaust piping and the black "brace" under the car? It seems a lot of people have trouble with the cats rattling at certain RPM's in that general area, and sometimes have to bend/bang the heatshield a little bit for more clearance.