AC screeching noise / 92 100 CS

[Licubar@aol.com]

I have an auto climate control system.  After driving w/ the AC on for about
1 hour ("normal" power, right around 72-75F) the AC makes a screeching noise.
 It is not a loud noise, but annoying.   The noise stops when we turn off the
AC, and turn it back on again....or, if the car goes over a bump on the road.
 

Before I take it to the mechanic, I would like to be more specific when
describing the problem. Any tips on how to "diagnose" the noise?

Linda