OP: who installed the Stillen G3? You or a shop? Could it be the holes were cut too big and the intake piping is rattling around?
The brace on the exhaust only holds the cats against the gearbox. If that brace was removed, it vibrates the whole car esp on deceleration. There are other brackets along the exhaust but none of which would rattle like mad unless they were extremely loose/not secured.
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