What will make a grinding noise that is related to the speedometer on a truck with a speed sensor system?

Begin by taking a closer look at the noise itself. Is it actually coming from the instrument cluster or is it just "under the dash" ? A short piece of old garden hose held to your ear will help you isolate the sound at its loudest point. Does the sound change (ie volume, pitch,etc.) with speed ? Is it always there or sometimes there ? Watch the needle as you drive and note if it holds steady, bounces or sticks at any point. Often, grinding noises associated with the speedometer are just the result of a worn drive cable. This runs from the back of the cluster through the firewall to the transmission. Over time, dirt and moisture can enter through worn seals and wick way up into the cable. The resulting corrosion can lead to erratic operation, cable breakage and grinding or ticking noises. Inside the speedometer is a friction drive that raises the needle based on how fast the cable is spinning. Without this, the needle would spin completely around or snap off the first time you accelerate. Bouncing or an abnormally low reading for your visual speed would indicate wear