There are messages here and in the Technical Manual about the tachometer.
A screeching sound or wobbling of the needle usually means a problem with the cable.
Some suggest dropping some oil down the cable but I think most would recommend a new cable which is available from several sources.
While the Tach is out, might want to change the speedometer cable since it eventually will have a similar problem.
There are messages here or in the Tech Manual about how to remove the gauges.
Richard M, NYC