I would say it's the aftermarket belt. When the cheaper belts run their backs against a smooth pulley, they get buffed and glazed. When they "walk" across the pulley after they get glazed, they make that noise. I try to use factory belts or the pricier aftermarket belts without much printing on the smooth side. Belt dressing is just a temporary diagnostic tool, it won't cure belt noise.
Just my 2 cents worth,
Scott
