I would say that your belt "zing" noise was the lower run of the belt rubbing on the chassis tube below the bevel box drive sprocket when you accelerated, it runs with minimal clearance. The noise is the ridged outer side of the belt just touching the chassis tube. As you back off the power the slack in the belt lower run reduces and the zing goes...or you can tighten the belt and have a bit less slack, as you have done. It is a good clue that the belt tension is too low.
