My compressor in the ea was noisey all the time and it was the bearing for the pulley that was the culprit. Replaced that and haven't had a noise problem with it since. Does it sound like a noisey bearing? Mine was noisey for startup of the car to switch off. As long as the motor ran, i had the noise.
Also, when it screeches, does the compressor stop? It could be that it's starting to seize....
Good luck is all the advice i can give you now!
EA Fairmont Ghia : 3.9L MPFI, 2.5" cat back sports exhaust, K&N panel filter. Pioneer head unit, pioneer 4" three way fronts, Pioneer 6" 3 way rears, 2x 12" subs, blue interior light.
My main pride and joy : '76 XB Fairmont GS, 250CI, 140,000 Km and all stock.... for now! (Factory air-con on the way)