Grinding noises in my instrument

TrailBeast said

Yep.
Slide the cable out of the housing from the speedo side, clean and lube it then put it back in.
The housing is rubber on the inside and when the cable gets dry it sticks and releases causing the noise and the needle jumping around.
Think about it like this.
The cable sticks and winds up a little and suddenly releases causing the cable to turn the speedo faster for a second.

Click to expand...

Actually its more like inertia that causes the needle to twitch. The input needs be smooth rotation. There is a spot underneath the drivers seat where the cable turns a 90 degree curve. That's where the cable normally binds in the sheathe. Pulling the cable around that curve to remove it is easy. Pushing it back in around that curve , not so easy.
If the cable is disconnected from the trans , set out of the strap in the curve and straightened a bit first... Disconnect at the speedo allows chemical cleaning/compressed air flush of sheathe and cable and lubricating.