Tire Hum or Wheel Bearings?

My '88, 420 SEL, had developed a low frequency hum that sounds like a tire but after researching the subject, here, it looks as though it may be a wheel bearing. The sound seems to come from the front passenger side. I'm going to have the tires rotated and if the hum continues then I'll assume it's bearings. It seems that I've read that some tires have a wear indicator designed into the tread but I don't know if that's a visual or audible indicator. I'll appreciate your comments and advice. BTW, the hum is more prominant at low speeds and become nearly inaudible at highway speed (70-75 mph).
Jerry

[BenzOnline]

It is most likely the bearing because mine did the same thing and it came from the same area. It was the bearing.

While trying to avoid third-degree burns, drive your car for awhile, then with your hand (carefully!) feel the hubs of both front wheels. If you can detect a difference in their respective temperatures, chances are something (like a bad or dry bearing) is generating more friction, and thus heat.

This is by no means a definitive test, but give it a whirl.

[Herr Leber]

It is sometimes tricky to determine if the wheel bearing is worn. By doing some extreme swerving left and right while going down a preferably deserted street or parking lot you can listen to see if the suspected bearing gets quiet under hard loading. Also under hard braking the bearing should quiet.

Could it be the differential or driveshaft?

Thanks for the replies. I'll try your "tests." This car has only 80k + miles. It seems odd that a bearing would be drying up at this relatively low-miles point. If I discover that one bearing needs replacement should I assume that all the wheel bearings need replacement?

[Herr Leber]

Ya never know. If the seal goes bad, all the grime that gets in there will eat the bearing up. Add in a little time and rust and before you know it, youre doing the bearings.

The previous owner could have hit a few major potholes somewhere along the way also, as to speed up the bearing aging process.

I would pull the hub off to take a look. You'll have to take the caliper off and hang it on the side with no load on the brake hose. Clean it up spindle/hub/bearings, repack it with fresh grease, and set the pre-load on the hub nut. (no load-no play).

Thanks, Herr, I'll give the hub a look-see, and repack the bearings. Or, if it seems that there's a lot of crud in the asseembly I may just go ahead with a replacement.

I tried the "hot hub" test. The front passenger side hub was slightly warm while all the others were cool. I forgot to do the swerving test but did do the braking test. The hum stoped when I braked but started again immediately after letting up on the brakes. I'll focus on the front passenger wheel. On the Mercedes Club forum one responder said that he had a similar situation as mine and had the bearings repacked at a Mercedes dealer and the hum was still present. It turned out to be a brake caliper touching the disk.