First two cheap things to try, even though I don't like shotgunning (but this is a tough one to troubleshoot) is to simply change the belt to a different design. Gatorback if you don't have one, a good Gates if you already have the Gatorback. Take a good look at the idler for the SC belt as well. Yours should be well worn by now, and that can transmit vibration through the entire belt path as 'chatter'.
From there, we'll dig further, but that'* the best I can give you for right now.