I know the noise you mean as my 1990 w124 300d non turbo (UK) car does it.

Mine sounds as if the auto box is whirring,but it is not though.The noise appears to come from the gearbox area but the car has not got the wind tray fitted any more so it is hard to be certain the area it does come from.

I reckon my tensioner pulley wheel bearings are bad but have yet to investigate properly,I can alter the noise which I suspect is bad bearings in the wheel by pressing on the tensioner quite hard at idle

Now I changed the leak off pipes at the weekend and while running the engine to check for leaks I came across a noise which was similar ,this noise could be heard at idle and slow driving.It was a tinny rattle and had been driving me mad.

The cause was the first 3 injector pipes from the front of the engine.

There is a bracket they clip to and they are held in plastic clamps,well sometime the injector lines have been off the engine and the lines had been replaced with the clamp already in the bracket so they sat on top of the clamp they were supposed to be held in,just slightly above the plastic but close enough to rattle and vibrate on it.

I had my hand on them with engine running and felt vibrate,so I placed my fingers on all 3 and the noise went,so unable to remove the plastic clamp I just used two cable ties for now to hold them down to it.

I was very shocked at how delicate the injector lines are compared to other diesels I have owned and I suppose the thickness of them lends itself to picking up vibrations.
Anyway they were extremely noisy and would get louder as engine revved until at around 2k revs the engine noise drowned them out.