This noice worries me
 

While driving on the highway a noise comes and goes from the engine compartment. It sounds like a fan or motor revving up. More like a blower fan. It will disapate after a while, there is no set time span, and then engine sounds perfect. As far as I can tell there is not any change in engine performance, handling, or anything else.
The only thing which has not been replaced or repaired is the power steering pump. Could this sound be an indication of impending failure. It has been going on since May and I have put about 5000 miles on the car during this time.
There is a leak in the steering gear box and I have to add fluid often.
Any suggestion would appreciated.

Just the aux fan turning on? Are you above 212°F or running with A/C on?

In the past the aux fan power was rerouted, therefore it is always on when the a/c is on. I plan on reconfiguring it to its original setup in the near future.

Sounds like it could be the clutch fan working as designed...or not working as designed.

If the clutch fan is failing how would you know?

Clutch fan failure is signaled by the engine getting hot at traffic lights. if it's making a noise, turn the radio up louder.

Yeah, I'd suspect that viscous fan clutch too. Power steering makes a very characteristic moaning noise that I'm sure you've heard coming from beaters in parking lots before. It'd be strange to hear one on the highway only for a few seconds and not at low speeds while turning, but I suppose anything is possible. You could always drive around without a fan at all to see if that's the case ;-)

I think I have exactly the same problem with my 4.5, and in my case it can't be the primary fan or fan clutch because I have eliminated those parts and replaced them with an electric fan.

It sounds like a power steering pump that is low on fluid, and the gear whine sound is both intermittent and limited to a rather narrow RPM range that corresponds with 50-60 MPH or so in high gear. But alas I am unable to hear any noise from the pump using a stethoscope when I rev the engine at rest to the requisit RPM. It does seem to be coming from the engine or its accessories, as reving it in neutral while driving produces the same sound.
It's driving me crazy as intermittent problems are inclined to do, so any advise other on the forum have would be much appreciated by me, too.

Drive around without power steering? :(
Hahaha, but first sign a document absolving me of all blame. Well I s'pose it's not the tranny if it's doing it in neutral, how about the timing chain slack? Do you have an A/C compressor?

To be clear it is not sound which last for a few seconds. It will last up three or four minutes. The noise has a low pitch to it.
At first I thought it was the a/c compressor kicking in, because it sound a lot like one. It is not the a/c.
I have tried to stop and locate the noise but am unable hear it or it goes away at lower speeds and/or at a stop.
The engine operates at normal temp. and the timing chain is new. Next time it makes the noice I will shift to neutral to rule out the tranny.
At idle what noise would a failing fan clutch make?

Well it'd just be fan noise if anything at all. A low roaring fan, it'd be noticeable when revved just sitting there actually. Sort of like a big rig in a traffic jam. They either fail stuck-on or totally freewheeling, which was the failure mode for mine so I heard nothing. You'd be able to goose the throttle rod under the hood in your driveway and hear and feel the fan blowing ridiculous amounts of air back at the engine.

Actually this is a very difficult thing to test properly. I think they have a high-rev disengagement too, in addition to the temperature engagement, such that they don't spin on the freeway. Perhaps that is what is failing. I feel like it is a long-shot though. If you removed your fan temporarily you'd be able to rule it out though for certain, it's only 4 little bolts, just don't get caught in traffic!

I think I will defer to the Zen-benz masters on this one.

I pulled the fan clutch off the parts car and replaced it with the one in the SEL. I have put over 100 miles on it and have yet to hear the offending noise. So it looks like problem solved.
Thanks guys for you suggestions.

Good to hear it Britton, while you're here, maybe you can tell me how to get the subframe mount out of my car. I've been going through your excellent guide in your thread and I've gotten to the point where the spring is out and everything is taken apart. The upper arm is still in place however. I removed the 4 small bolts on the mount, do you need to loosen the large center bolt too? Is that even possible? With the 4 bolts out the mount seemed to drop an inch or so but the subframe didn't fall enough to get it out. Any suggestions? Perhaps I have to remove the small "leaf" spring too? (after adding a jack under the engine)

If it's anything like a W107, you do one side at a time and undo the top bolt to the shock. This will drop the sub frame down just enough to remove the sub frame mounts.

On a W108 and others that use the same floor pan,the easy way is to raise the car up in the air,support the body under in the inner chassis rails and have the jack under the crossmember.undo the shock absorbers from the top .
Undo the brake hoses and the steering ends off the steering box and idler.Remove the crossmember locating rod. ( budget for a new one)
Mark the front leak springs so you can locate them back in the same spots and undo the sway bar bolts .
Undo the big center blot each side on the mount and undo the four securing bolts and lower the whole lot down with the engine and trans to a point where the mounts are clearly visible. Replace the mounts and grease the large center bolt. screw it home so it is still sligthly loose.
lift the whole deal back up and locate the four securing bolts on each side .Once they are in ,replace the front sway bar with new bushes .
Set the front axle on jack stands on either side to replicate a load situation and tighten up the mounts bolts and the sway bar bolts.
reattach the steering and shock absorbers. Replace the brake hoses and bleed the brakes.
Done.
It's quicker and easier if you have a jack suited for the job.