Speedometer Lube
From the FSM:
SPEEDOMETER CABLE
To service a noisy speedometer cable, disconnect
housing at speedometer head. Remove shaft and clean
it thoroughly. Apply a very thin film of speedometer
cable lubricant on the shaft. Such a lubricant is available
under Part Number 1243632, Speedometer Cable
Lubricant. Wipe excess lubricant from the top one foot of cable and ferrule.
FYI the lubricant specified is Lubriplate ( White Litium Grease)