Help please !!

I tried a 3MScotch-Brite pad on the smooth side, while it was idling. Had to polish the smooth idlers as well. That worked for quite a while.Your results may vary.
I recommend to go in shirtless. Try not to lose any body-parts.
Eventually I had to replace the idler...... so did the belt as well. It was all quiet after that.

I think the noise actually comes from the ribbed side sticking to, or squirming on, dirty pulleys; because on one job, even a new belt squeaked, until I cleaned the pulleys. I found a lot of rubber transfer.
On another job it was anti-freeze residue on the pulleys.