I had this problem in a '79 300D and it turned out to be water in the speedo cable had turned to ice. I pulled the cable out of the car, hit it with a hair dryer to get it really warm, let it sit for a bit in the hope that the water was gone, then lubed it. Noise went away.

Phil Forrest

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1972 220D "Trudy," named by a friend.

"The 220D sounds good... I suspect it is the only car that you need a calendar for, rather than a stopwatch, when doing acceleration tests."
Tom Abrahamsson