Broken speedometer

My speedometer makes a grinding howling sound when you get above 10mph and then the needle fluctuates. How should I go about fixing it. 1970 dodge challenger

The cable not be fully engaged in the speedometer head?

Try this........

Take it loose at the transmission.

Spin the inner cable slowly by hand while pushing it upwards.

Re-attach to the transmission pinion & check it.

Ok I will try thanks! It always worked....but the car sat for a year or so without being touched. The first time I tried it out after sitting it acted up

If that doesn't work try removing the cable core from the speedo end of the cable housing and lubing it. I use a very thin coat of either white lube or Vaseline

I'm beating on this one

Ok thanks!

You might spin the cable with a reverseable drill (CCW) & see what develops.

I had this happen on a motorcycle and it was due to the cable not being lubricated. It ended up trashing my speedo.