Sounds fairly like a cooper S driving about.
At first I thought gearbox or driveshaft, but there is no vibration, at least no more than usual, and its linked to the engine speed, so it does this in neutral too.
I was guessing maybe the alternator is on its last legs?
Anyone slightly more mechanically minded like to hazzard a guess?
Cheers
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2005 Peugeot 206 GTI 1802008 BMW K1200R