I have a 1990 300E and its a 4matic. There is a humming or whining sound coming from the rear. It kinda comes and goes as I push and release the accelerator. My 1999 4matic never made this sound. It has 90,000 miles which is pretty low for a 90'. Could this be the normal sound of the fuel pump? Or is my rear end about to blow up. Its not super load or anything, but it is noticeable. Thanks guys. Its the 3.0 motor. solid but ohh so sloooowwwww

If it's doing it in neutral or park, I'd assume fuel pump or exhaust system. If it's in drive (car moving) I'd guess differential.
If this car is really really slow, make sure the throttle valve cable is connected (under the air cleaner).
Gilly

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Hi guys,
my car 1988 300e is doing the same. I noticed that when the fuel is running low, the humming is louder. But if I fill the tank up, the humming dissappears. I think it has something to do with the fuel pump intake or fuel backflow inside the tank. Because it didn't harm the performance, I don't do anything about that. I hope milhan's problem is the same. By the way, for your information. I changed one of the two fuel pumps before.

The diesel version of these 124's also have been known to make the same or similar sounds including my 87 300D. Diesel 124's have no fuel pump in the fuel tank or anywhere back from the engine.

At approx 50 mph, I can begin to hear a hum that will rise or fall depending on how much I push the accelerator. The more throttle, the louder. Let off the throttle, the hum softens to quiet. After reaching 55 mph, the hum sound dissipaits to no sound at all. The hum is only there between 50 and approx 57 mph.

Sound familiar?

Possible causes may be worn wheel bearing, or worn or loose differential ring bearing.

I have driven 30K with the hum and it has not changed at all, and I'm not worried about it. If the sound gets louder, or spread over a wider range, I'll pay it more attention. BB

[IMG]Both the 190e and the 300e have differential whine that is only noticeable on the highway, they both have over 230k miles on them. The telltale is the lifting of the gas pedal and applying it.

I started using 90w-140 differential oil in the 190e and it seemed to quiet it down a bit.

I read also on this site where someone recommended Redline's synthetic diff oil. I haven't tried it though.

Haasman

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I forgot to mention that I have changed out the diff oil with Moble 1 synthetic diff oil, (sae 75-90 I think) and no changes but there is a little leakage, not bad, but I do keep an eye on that! BB

This happens on my 87 300E from time to time. In my car, it also gets louder when you step on the gas, and then seems to quiet down when you release it, although sometimes it can get very loud without any pedal movement, and I've had it show up when the engine is off (!) too.

I discovered a little while ago that if I stop the car, walk around to the rear, and crack open the gas tank cap for a second, it seems to depressurize the fuel tank and the whining stops. I don't know what this means, or why it does that, but thought that it might be helpful.

Interesting, I will try that. It sure is annoying.