Regarding my '80 245 DL with a M-46.

My car has recently developed a rather loud deep rumble / scraping noise that is coming from the rear end of the car. It comes and goes at random intervals, and speed doesn't seem to make a difference.

It only happens when the car is in motion. When stopped, the noise is gone.

Putting the car in neutral and coasting has no effect on the noise either.

Rolling down the window, I can certainly hear the noise from outside the car. It is much louder, but I can't really tell exactly where it is coming from, except that it is coming from the back of the car.

I looked under the car, but I couldn't see anything out of the ordinary.

I checked my tire pressures, and everything is in order.

Any other ideas?

Thanks as always.

(OT, but my car will reach 440K miles this week! Well, that is assuming that I this problem can be solved quickly.)
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If you listen to the radio in Portland, OR, you may know me as 'Portland's Favorite Soul Brother!'