Screeching is almost unmistakably due to slipping belt:

1) Check belt tension - if already tight then replace with new belt, the old one had glazed. If its still new belt then readjust tightness but do not overtighten. New belts could stretch a bit after several heat cycles especially if installed borderline tight.

2) Use metric sized belts - this is a MUST. Non-metric belts can't have optimal contact (gripping) surface to the metric sized pulley. Microslips could happen then suddenly one day screech!

3) If the AC compressor belt still screeching (after doing no.1) - you'll have to think further up than the belt. There might be issues with the compressor. Its pulley bearing might be on its way out or the compressor itself is failing with extensive wear inside. Prepare to spend more.

4) If the power steering pump belt still screeching (after doing no.1) - the Saginaw steering pump has a high pressure bypass valve to divert excess pressure when steering reaches lock to lock. This bypass valve may be blocked with crud/wear particles. The pump pulley will LOCK if this excess pressure not relieved. Dismantle the high pressure hose at the steering pump to remove the valve for cleaning.

Refer to this GM website for more:
http://www.gmtruckcentral.com/articles/2013/powersteeringmods/

(the website is more about modifying steering pump pressure to provide more steering assist but you'll see where the bypass valve is located)


Regards,
Amarin.