Originally Posted by Gumby22

Not an expert - I’m an auto tech by trade and I have several customers that either want brake controllers installed from scratch or have problems with the controllers they already have. I’m constantly referencing wiring diagrams for several different makes (though GM is by far the largest part of our customer base) to diagnose controller and/or trailer wiring problems, so I’m often looking for solutions like tapping into the fuse block instead of the brake switch.

I’m yet to see a 2020 express, but last I knew, the primary drive belt still uses a tensioner. The AC compressor belts (on the trucks and vans at least) have been using a stretch fit belt since at least 2014, so I expect that hasn’t changed.

Try cycling the AC on and off to see if the squeal changes in pitch or goes away. If yes, focus on the AC compressor and belt. If not, remove the primary drive belt and spin the pulleys one at a time to check for noise.