Sounds like I have the same problem - I picked up a 2009 M3 convertible with 35,000 miles a few weeks ago and was getting the same noise @ ~40-50 mph, on load, in any gear. Luckily it was almost inaudible with the top down, but quite annoying with top up. Local dealership replaced the diff fluid which did nothing and then had the car for 8 days but couldn't figure it out. Said it might be normal but didn't have another M3 to compare with.
Got fed up with them and took it to another dealership who just decided to order an entire new rear diff. Waiting for it to come in, but hopefully that will do the trick!
Unfortunately I didn't see this thread (or really any other thread) describing this problem before bringing it in. I described the noise as almost a muted "howl", but it seems like very few owners notice it/post about it. My advice: take it to another dealership if yours is refusing to acknowledge a problem.
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Euro Delivered: 15 428i xDrive Gran Coupe M-Sport EBII
Past: 12 VW GTI, 09 E93 M3, 07 E92 335i, 06 WRX STI
Last edited by TurboSpecC; 06-08-2012 at 10:01 AM..
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