The old 300SE sounds like it haunted
 

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

Power steering pump?

Could it be engine mounting related?

Have you checked the bearings in the distributor? My 560SEL sounded like that right before the distro went out.

Argh, that's one of my dumber suggestions since the M103 doesn't have an actual distributor per se, just that cam shaft attachment. My next guess would have been water pump, but you said you ran it without the serpentine...

I'll poke around in my 300SE when I get home and see if I can think of anything.

Well, I looked my 300SE up and down last night, and I am at a loss as to what could make that noise from the front end, even with the serpentine belt off.

Unless it's timing chain related, I'd still pull off the distributor and check that the back cover, the red plastic disc that "seals" to the back of the distributor cap hasn't broken or otherwise found itself scraping against the camshaft. There's also the camshaft seal on the timing cover, but I'm really grasping at straws here.

One thought I had was that your EGR is leaking and making an exhaust whistle, but I have to think you'd have located it if that were the case.

I mean, you took off the serpentine belt, right? What is left spinning after that? The cranksaft, the camshaft, the oil pump, the distributor, and that's pretty much it, no?