Odd squeak/quack from left rear wheel?

[Trotskyite]

I started hearing a rhythmic squeak from my wheel yesterday. The car is an '82 SC with 107k. The tires are Falken Azenis RT615s with about thousand miles on them. Here are the symptoms:

The sound is somewhere between a quack and sneakers on a basketball court, i.e.. definitely rubber/plastic sounding

It only appears off throttle, but not when coasting.

It speeds up/slows down with wheel speed, not engine speed.

It only seems to be coming from the left rear side, not the right.

I am getting no abnormal feedback through the suspension.

I took it in to my Porsche shop this morning and they had no clue. I took one of their very experienced mechanics out in it and while he got a kick out of it, he had never heard anything like it. Has anyone had a similar problem?

[DRACO A5OG]

Rear Torsion Plate Bushings? Is the end cap in the center of the plates or rubbing against the plate, a pic would be helpfully.

[carnutzzz]

Roll backwards slowly, pull up e-brake to stop.

Report back.