Sounds like I have either a ghost or a small dog howling at the moon in the '91 300SE.

Coming from the front of the engine when cold at fast idle; goes away as engine heats. Also hear the same thing (I assume) when driving at 2500 in any gear; moans under load, stops immediately when I let off the gas.
Not too loud, but noticeable. Done it for months, no apparent detrimental effect - YET.

I know what it's NOT:
- NOT THE FAN CLUTCH - took the whole fan/clutch assembly off and it still did it.
- Not the air smog pump - disconnected
- Not coming from any of the fan-driven accessories - took the belt off, still did it at cold fast idle.

Anyone ever had one of these "haunted" 103's?

DG