I am having a similar problem with my '82 wagon. It sounds like there is a screwdriver smacking the inner wheel well with every turn of the wheel (i.e. it is faster at higher speeds). If I had to use a word to describe the sound phonetically, I'd say "clink".

Sometimes I think that if I were watching the car from outside as it made this noise, I would be able to see the onomatopoeia, like in the 60s Batman series.... "CLINK!" in a big spiky bubble...

Anyway, I think the problem is the CV joint, from what I've been told. Since its coming from the rear end, I checked the differential for metal debris in the oil, and it was fine. The only other thing that could make this kind of noise is the CV joint. I'll follow this thread and see if you find the soution to be something else, before I go order an axle. Good luck!

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'82 300TD
'77 6.9
'75 280S
'74 280
'87 Porsche 944 turbo