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Mine has always sounded a little bit like a diesel on idle. Been to three dealerships who all said sound was normal. Remove the bottom of your airbox and place a K&N air filter on it and when it revs you won't care about the tapping.
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 E90 320i 07 Exec | innovations package | comfort access | 18" ellipsoid 162 | Yokohama 225/40 front 255/35 rear |