On mine it was the worn needle shaft lower bearing. At the lower end of this shaft an aluminium pot is mounted inside of which a solenoid is rotating. The lower bearing, the one near this pot, wears obviously quite quickly, becomes oval. So under vibration the pot no longer is sufficiently centered over the rotor inside of it.
I ignored this until finally the pot got contact with the rotor, thus pushing the V11 nearly to warp speed, at moderately revs even.
Hubert