weird sound coming somewhere from the rearend

my '00 E320t 4Matic with its 104K miles mark is getting expensive and expensive ;-). I discovered a new thing - there is a high pitch coming
somewhere from the rear end. It is not loud but noisy enough to make it very distinct to hear. Maybe not that high at the frequency but remains constant regardless the gear, speed and rev.
Is it possible it is a fuel pump. I had just a week ago a service B performed and IIRC the fuel filter got replaced.
Any clue what it could be?
Thx, Andy
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