My 2011 Enclave is blowing hot air and making an intermittent horrible screeching noise. So far I have checked the low side pressure and it was very high. I don't currently have a way to check the high pressure side. I can see the clutch go in and out. It seems that when the clutch engages the noise starts and lasts a couple of seconds then goes away. the pressure doesn't change when that happens. I plan to attempt this repair on my own but I don't want to just start throwing parts at it. Before I do anything I thought I would ask here first.

Ugh! I think your compressor may be toast. Sounds like what I just went through with my 2013 LaCrosse, which I just brought home from the shop a few minutes ago. My A/C suddenly started blowing hot air last Wednesday. Stopped in at Costco and it was working fine and came back out a half hour or so later and it wasn't cooling. They basically had to clean house and start over. Not pretty and not cheap but at least it'* blowing cold air now.

90% chance this is the air conditioning compressor clutch going bad. Figure out where you have to be to see it from above (between the fuse box and engine cover, just above the coolant reservoir). Start it up with the air conditioning turned off. Go stare at the compressor pulley and clutch. Have someone turn on the air conditioning. What I'm guessing you'll see is the clutch plate moving slowly compared to the pulley, and your noise concurrent with that event. If confirmed, replace the compressor. It'* not worth it to replace just the clutch then have the compressor fail a wek later.