Please, does this ring a bell with anyone?

About once or twice a year our car -- a 1992 245 with 250 k miles -- generates a metallic BANG-BANG-BANG from the rear.
It's loud. Not quite end-of-the-world, but close. A "Something is wrong" noise.
It varies with road speed, not engine RPM.
It may continue for tens of yards, or a quarter of a block, I suppose.
Then it stops!

I suppose it bangs with each turn of the rear wheels, and not 3-1/2 times as often, as would be true with a driveshaft yoke-universal joint/something issue. But I wouldn't swear to anything.

It has only happened in cool to cold weather, say freezing or below.
And my wife thinks that humidity may promote it?

My ignorance of the workings of the entire rear axle assembly is nearly complete. I can't even guess at anything.

Is a parking brake shoe able to come adrift and cause it? (I did take a broken one out of our 745, which had caused no noises.)

Thanks for your help!

(Written from Minnesota, where the temperature has gone below freezing, and snow has come, and I want NO problems for several months to come!)