My ī91 W140 400 SE suddenly adapted a strange tapping noise a couple of months ago. The noise is not present when the car is cold, but occurs after 20-30 km and increases in intensity the longer I drive - and only at low speeds, below 10-15 mph or so.

Itīs hard to tell where the sound comes from, but it seems to come from somewhere in the front or middle of the car.
The tapping seems to be related to the speed of the car, not to RPM. It also doesnīt matter if itīs in Drive or Neutral.

Any ideas what might cause this?