The infamous belt squeel... for me, it was bad engine alignment from the factory. Re-aligning all, engine (thus primary) and rear tire alignment (thus pulley) did the trick. It wasn't easy but just in case this might be your issue, one test that really helped me was to look carefully at the position of the belt on the rear pulley- is it running close to the inner or outer flange? OK then, now on a level surface while in neutral roll the bike backwards 10 feet or so (enough to let the belt travel a few full rotations) and watch to see if the belt moves from one side of the rear pulley to the middle or closer to the other side. If it does that, that the rear drive needs better alignment, and it might include getting the engine and primary better aligned. Mine was not forgiving at all... I had to get it perfect to get the belt squeel to stop.
Now at one point I raced a .25 mile against a big mouth dood with a porshe. It was nice to win, but I ended up pulling the rear pulley just slightly out of true- yes, most of us (even old dogs like mine) have that kind of torque. So depending upon how hard you beat on your bike that may be a possibility too, your pulley may not be straight anymore.
Just food for thought to add to the rest of the good advice being offered. Good luck!