I wonder if you are hearing the fuel pump?
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Crashton Club Coordinator
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BThayer23 Well-Known Member
Could also be a broken exhaust hanger.
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^^
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There is a pump in the left rear for the vapor recovery system called a Leak diagnosis pump. It is possible that might be the source of the sound as well.
Diagram found here RealOEM.com  BMW R50 Cooper Activated Charcoal Filter/mounting parts