Update
The dealer called and said that they think it's the differential, so they've ordered one from Germany and it should be here in 7-10 business days. I'm not convinced that this assessment is correct, as the 2012 loaner with 5,000 miles on it that I'm currently driving makes exactly the same high-pitched whistling/whining sound at the same speeds. Aside from the delay, however, I don't much care, as I'm happy to get a new diff on my 46,000 mile car for free.
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2011 328i Coupe | Alpine White | Oyster/Black Dakota Leather | M Sport Package | Premium Package | iDrive | Harman Kardon Surround | Steptronic
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