One experiment I did was to buy a fractional horsepower belt at Grainger. They are made out of different material and had no cogs. People say cogs are not good, but those are about the only auto belts you can buy. The one I bought was 3/8 width, so it wasn’t the same width, but worked. Squealing stopped. I don’t know if they are rated for auto use, so I was a little unsure about running it long term. I sanded my auto belt after that and tried again and it didn’t squeal anymore. Sorry that didn’t work for you. I chased this forever with no luck till the end.
