As for synchronous drive belt slippage, if there is ANY significant tension in the belt at all, it doesn't and can't happen unless teeth are shearing off. What you are likely experiencing is belt 'slap' and/or 'snatch' from rapid load changes changes caused by throttle inputs and/or shifting. These types of events can be somewhat violent and extremely noisy, especially if you have a machine with the ill conceived factory NVH 'upgrade'.
Tightening the belt is not the answer. There really should be some sort of tensioner/anti slap device on the slack side of the belt. I believe someone on this forum has posted their installation.
