Just a fwiw, I ran into this squeak thing and I tried the usual 'fixes'....eventually I coughed up the cash for a GM belt vs whatever the parts store had...the squeak went away.
I observed that the parts store belt felt like the rubber was hard vs the GM boxed belt in my case. This I thought very strange, old stock sitting around a long time?
I just don't know what to say, but you could feel the difference between the two belts!
Tom