[TorqueHead]

Hoping someone can help me out.

My 92 C4 has a consistent thumping noise coming from rear wheels when turning left or right, even in a very slight turn.
The thump frequency is tied to speed. Faster speed, faster thumping. If going in a straight line, no thumping is heard.

Backstory:
Car is new to me and was stored for about 15 years. I have replaced all fluids except transmission and front differential oil. Those are next on the list. Diff locks have been flushed with UDT999.

Car is now back on a lift with all 4 wheels hanging free. When I rotate a front wheel, the other front wheel will counter-rotate. When I rotate a rear wheel the other one always spins in the same direction. Is this normal?

When I rotate the traction control knob clockwise, I get one fairly quiet knock from the diff lock slave but when I rotate the knob counter-clockwise, I get a series of 3 or 4 knocks that start loud and then get progressively quieter. Regardless of knob position, the yellow light stays on. This sounds messed up to me, but this is my first Porsche so I turn to the Rennlist community for guidance. It seems like maybe the transverse diff lock is stuck in the fully locked position??

I have checked for play in wheel bearings and CV joints and all looks good. I can slide the half axles in and out a small amount and they slide smoothly. CV joint boots look ok.

(also posted on Rennlist)