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Your muffler should have a vacuum actuated butterfly valve that blocks the exhaust on one side at idle. It opens up under acceleration, or when the car is off. Many people block off the vacuum line so that the valve remains open at all times. Search "golf tee mod" for hundreds of threads on the subject.
04 IR 330i ZHP | ESS TS2+ SC | MagnaFlow "headers" | KMS 3.64 LSD | AST 4100 550/650| APEX ARC-8 18x9 | H&R front sway bar | Vorshlag camber plates, RTAB limiters | TMS subframe & swaybar reinforcements | UUC SSK & DSSR | UUC TM & TME | BW motor mounts | Dinan strut & shock tower bars | vaio 4-bolt oil pump