Have you checked your belt deflection? The squeaking is just at an idle, but quits when you give it throttle?
Belt deflection is how high your belt sits in the secondary at rest. This is adjustable by three bolts in the back side of the secondary. The farther the bolts go in, the more it holds the sheaves apart and lower the belt sits. The bolts have washers that can be added or removed to adjust, but the ends of the bolts also wear and need replaced over time.
The squealing is from the ends of the bolts wearing down, which then allows the sheaves to sit closer together and makes the belt sit up higher in the secondary. This pulls the belt tighter on the primary and can make it squeal.
Remove washers from the three bolts, and it should go away. If it doesn't, then replacing the bolts would be a good idea, start over with washers on them.