Dave,
In my 30 years as an MB mechanic, I have relpaced very few cables. MB cables just never seem to give up, but the speedo head on the other hand generally makes the noise you describe. It gets your attention when it starts doesn't it?
You will need to get it rebuilt. If you choose to go the breaker yard route, make sure the code on the back of the replacement unit matches or you could end up with a speedo that doesn't show the true car speed. Unlike American cars where the calibration is done with a gear in the tranny, MB speedo cal is done in the speedo head.
Peter