I have had this problem twice. In every instance, the drive belt needed adjustment. In one case, the belt was loose enough to rub on the small, mud deflector just forward of the rear tire.
Second time the side-to-side adjustment was slightly off, allowing the belt to rub on the side of the tire.
In all cases, the squeak started on throttle lift.
Adjust the belt so it has 10mm travel when you press in the middle and has a decent gap between it and the tire...still tracking so it doesn't rub on the sides of the rear sprocket.
