I have a 91 GL that has developed a "squeaking" noise in
either the cable as it attaches to the speedometer, or the
speedometer itself.  The noise interval is directly related
to the speed of the vehicle (surprise!).  The noise
literally sounds like the cable is rubbing against something
rubber-like, which makes the "squeaking" sound that I can
hear.  The sound is quite distinct at low speed (little
engine and road noise to obscure it) but is less obvious at
higher speeds (more noise).