1996 Geo Tracker Squealing Noise when I turn the A/C on

For vehicles with A/C, when the A/C or Defrost is turned on, the A/C is engaged. The reason for the Defrost using the A/C system is because it is used to remove the condensation (moisture) from the windshield as when the A/C is on, it does the same but, with the humidity in the vehicle. It sounds like the A/C pump is working hard when the A/C clutch is engaged. I would reccomend having thebelt looked at for quality (which may not be good enough when the A/C pump is in use but good enough for the others without the use of the A/C in use) and if the belt is of good quality, have the pump looked at by a professional. It may be on it's way out or just needs a simple adjustment. Hard to say without looking at it. Be safe.