The noise comes from a vibration in the rotating magnet. It is caused by dry lubrication and too much free play. Technically the repair is a mount with bushing (they are probably relatively cheap), lube and adjustment.

I fixed these for years before we actually got technical and started real speedo repairs. The lube and adjustment will be satisfactory in all but a few with really worn bushings.

Remove the speedo head, remove the chromed back plate and there are two screws that hold the magnet housing/magnet assy to the back. Remove it and you will see an adjustable tentioning block against the forward center of the rotating magnet. The head of the magnet dancing is what causes the noise. One should remove the magnet from the housing, clean and lube. On reassembly adjust the endplay with the thrust tentioner above. Unless you have a 110 body with the revolving speedo, this is all pretty easy once the cluster is out.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician