So after an exhaustive search I decided to start a new thread with what exactly is happening..
I have a 96 blazer with the 4.3 vortec. When we turn on the ac it blows cold, but weak and has a VERY high pitched screech while on. Some times its a screech and other times it's a low hum. While inside the vehicle the sound seems to be coming from behind the control panel.
I'm really hoping someone can give me some direction here. If you can just tell me whats wrong I'll look up the tutorials on whether I can fix it myself, but just wondered if you guys (and gals) could help direct me.
Thanks for any help on the matter.

 

Another thing it does that I forgot to mention.. When I turn the ac on and off to try and quit the noise you can physically feel the truck jump forward! Like it literally shakes the vehicle! Not sure if that will help eliminate or decifer any problems, but thought it was important just the same..

 

I think the high pitched screech is the "weak" blower motor & the shudder when you shut off the A/C is the compressor &/or the compressor clutch. Something to look at anyway.

 

+1, could also be a vaccum leak

 

Okay thanks guys and, Cat, It's a coincidence you say that because I've been hearing constantly that it should be a vacuum leak thats also cuasing the truck to have a crappy idle, acceleration and drive.. haha