my W211 E280 CDI (facelift) emits a whirring/ whining noise a bit like an electric drill/ screwdriver, when i switch the engine off. this noise lasts about 10- 20 seconds and stops with a sound like a click. it can be heard from outside the car, and emanates from the engine bay. unable to locate the exact source in the the engine bay... any ideas? could it be the inlet port shut off motor? any other ideas? thanks in advance!