Here are some other possibilities:
Sometimes the steel insert in the speedo that the cable goes into has rusted a little. This makes it be hard to turn, which in turn makes the cable wind up then unwind fast. WD40 will sometimes fix it, if you run it with a drill while spraying it.
Another possibility is that the magnet has come loose inside the speedo and is contacting the aluminum bell.
You probably need to unhook the speedo cable until you get this fixed, as you run the risk of breaking the speedo needle.
If you determine that the speedo needs service, we can help you out at
www.rt-eng.com.