As far as I can make out this is a common problem with Harley Davidson belt drives. It seems correct tension and rear wheel alignment are very important to eliminate belt noise. Keeping the belt clean is also very important and Harleys have a lower guard to help keep grit and small stones from being trapped in the belt.

Some recommend spray on silicon lubricants that leave a dry but slippery finish, but it seems that does not last very long. Clearly any "wet" oil or wax based lubricant should be avoided as that will carry dirt and grit which would quickly damage the belt and the pulleys.

Peter

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