I've just noticed that my right exhaust has this same problem (2007 4.8i). The left one is fine. Since it is a lease, which is up in a few months, I haven't decided whether to get the dealer to fix them.

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2012 F10 550xi Carbon black/oyster
2011 E92 M3 SSII/black
2012 W166 ML63 Steel grey/porcelain
2007 E93 monaco blue/beige (retired)
2010 E70 35d Space grey/saddle brown (retired)