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Both are BS answers.
Do you hear a knocking noise or do you feel it? There's nothing special about exhaust systems. If you only changed the catback and nothing else, you have two sources of noise:
1) exhaust leak from either gasket, or premature failure of welds (I've seen it plenty of times before on brand new exhausts)
2) exhaust is making contact with the car somewhere, either from poor positioning of exhaust, bad hanger placement (bad jig or weld), or broken hanger causing excessive movement
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