I had a similar rattle or buzzing sound (between about 2000-3000 RPM)
in my 2005 M3. It also happened when I was alone in the car or when
my 100 pound daughter rode in the front passenger seat. When anyone
heaver rode in the passenger seat it didn't happen.
I usually have the passenger seat moved all the way to the rear. If I
move the seat slightly forward the sound is never heard.
Maybe it is a ghost...
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