[rotorwrench]

There is generally a small hole in the shaft bore of the speedo input housing near where the cable guide screws on. It has a small cotton wick for oil service and may have a small metal soft plug in it that can be removed. If the bearing gets sloppy on the shaft or if the speedo drag cup bearings are worn then the drag cup can try to stick to the rotating magnet inside there. This will make an awful noise and can peg the speedo needle out. It likely just needs to be lubricated. Some of these old instruments can go for 50 years or more without lubricant but they won't go forever.