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