Speedometer gauge is not reading correctly keeps going up and down

Hello every one. I owned a 1991 mustang gt. My speedometer gauge making a griding noise when you move and the gauge itself keeps on flunctuating up and down and when you stop still reading 60 mile/hr. I appreciate if you can give me an idea what to do. Thanks everyone

Is it the Speedo that is making the noise or is it coming from under your car? There is a plastic gear that might be worn out at the tranny end.