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.
I will try that. Just so I am making it clear (cause again describeing sounds is just near impossible) it is not a chirp once in a while on decel, it is a constant squishy squealy sound when it is not pulling on accel. Does that make sense? Sort of like a blower does, sort of a whining sound. Theonly thing I can think of is like sneakers on a gym floor but constant. (Squeaky sneakers on a gym floor is what we used to have when we had gym class and Coach A hole actually made you run during class, for all you young guys who never had gym class).
Crap I wish I could describe it better.