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I have the studs from my headers/cats sitting in the garage, if you need them for the time being let me know and I'll send them down (I'm up in CT but get down to the City pretty often).
There is really nothing in test pipes which can rattle. They are only hollow tubes.
If there are no loose bolts on the pipes themselves (especially the top outside bolts at the headers which are near impossible to tighten all the way) are you sure they are not contacting anything underneath the car?
+1 There is really no way that a test pipes could cause the issues you are stating, as I said via PM I would double check the installation I am sure something is wrong, especially considering you broke one of the bolts, that could be your problem.
i know some people who isntalled test pipes the mechanic would push the y-pipe a little too much and rip that to cause a rattle. put the stock cats in and see if you hair some rattle or air loss too