JW,
Check out this
previous thread on your topic. This reply in particular may help the most.
Your problem sounds just like the problem I had in my 56 210. The needle would start bouncing around and making an awful screeching noise. Here is how I lubed the speedo in my car: (your truck is probably similar)
Remove the speedo assembly from the truck.
Where the cable screws onto the speedo, look for a little brass cup in a hole. Dig out the cup with a pick or small screwdriver.
Inside the hole will be a little piece of felt. Take it out and clean it with a solvent and dry it completely.
Put a few drops of machine oil on the felt piece, and a couple of drops directly into the hole in the speedo. (most folks recommend sewing machine oil for this, I used Hoppe's gun oil. The colorless oil. Not the Number 9 solvent.)
Replace the felt piece in the hole and the little brass cup.
Use a small drop of oil to lube any other moving parts. Wipe off any excess.
Then you can put it all back together and the noise will be gone.
56chev
Lots of great helpful tips in there for the majority of the maladies that affect our AD speedo's.
Just my $.02 worth, but sounds like you have a speedometer "head" that needs some cleaning/lube. There are also several notes about over-tightening the speedo cable connector at the speedometer end that can cause similar grief.
Let us know how this turns out please.
Dan
