I5First, are you talking V8 or I5?
I5
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I replaced the exhaust with Gibson split rear system and had muffler shop replace my header with the Orion headers. The muffler shop didn't tighten the bolts connecting to mid pipe good so it rotated and tapping on brace bar. Only used muffler shop because my exhaust manifold bolts were broken flush. I tightened the bolts from header to mid pipe after I rotated pipe back to have clearance and rattle is gone.Just had my magnaflow cat replaced under warranty and am experiencing the same issue....came on here to see if anyone else had