It brings an issue of flex between bevelbox and rear sprocket that the belt has to work with.
I am not sure I get your meaning here?
The NVH is a slab of metal which the bevel box is attached, the NVH is attached to the frame with bobbins meant to isolate the vibrations and quiet it down. Since the NVH can move freely from the frame (albeit a very limited amount) this will cause the belt between the bevel box and rear sprocket to have varying tension as the bevel box moves.
