Sorry I mis-understood, I thought you installed a used belt from the donor bike when I said check belt rotation. Don't use belt dressing like that sticky stuff some people spray on a V belt. You can clean the belt with soapy water and a scrub brush and after it dries use regular silicone spray and it will help condition the belt and quiet it down at least for a while until it gets a chance to seat into the pulley. That is, as long as the tension isn't to tight. I would rather have a little chirp on decel
than a belt whine from being too tight.