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In my experience, most aftermarket mufflers are much louder than the OE ones; that is one reason tehy are usually cheaper.
A failed CAT can contribute to noise, but usually will also have a rattling noise from broken internal ceramic bouncing around inside.
A vibration or resonance coming through teh body may mean that a component is touching the body somewhere from misalignment or worn hanger(s).
If the sound is louder than acceptable at teh tail pipe, however, teh cproblem is a cheap/ faulty/ performance series muffler.
For original quiet operation, OE BMW parts are the only way to go to be assured of factory sound.