Did you ever figure this out?

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1981 240D 4sp manual. Ivory White.

Unfortunately, no. A kind local Mercedes mechanic (that's been working on our cars since the late 70s) inspected and listened and was pretty baffled by the noise. His opinion is it was either the chain itself that was damaged, or a faulty tensioner, or a worn out rail. I replaced just the tensioner and that didn't fix the issue, so I have a guy coming out hopefully in the next week or so to help me do the chain and upper rails.

I have zero'd in on the noise coming from the valve cover, on the left side of the cam sprocket, above the tensioner assembly. So, the most likely culprit is a bad banana rail. However, I stuck a boroscope down there and it looked like it had plenty of material left.

If that doesn't fix it, I may just give up on this car. It looks and drives great, but the noise is getting worse and it drives me a little crazy.