When the belt is worn down you'll hear it skip on hard acceleration and deceleration. This is what it will sound like:
If your board is making the noises in the video above, follow the directions below:
1) Determine which motor's belt is slipping. Follow these directions to remove and replace the drive belt.
If you install a new belt and the noise persists, install a new belt on the other motor.
2) If your board has two new belts and the noise can still be heard follow these directions to evaluate your pulley to see if its worn down.