if it was banging on that brace piece, surely there would be a hole or crack in the pipe where it had been contacting by now? or at least some tell tale signs.
Do you have stock cats?
My other guess is that it's just resonance in the piping. Get the car on a hoist with someone holding the revs at which it always rattles, wear oven mitts and grab sections of the exhaust to isolate the origin
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