Polyrib belts make a whole different set of noises than the old fashioned vee belts, the noises can be caused by pitting or corrosion of the pulley surface, surface glazing of either the belt or pulley, contamination, small cracks in the friction surface of the belts and hardening of he belt material due to age or thermal stress.
Sometimes there are minor noises that are amplified by weird harmonic effects.

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