Copy/Paste from somewhere else
"That empty space inside it can echo noise, naturally. Too much of this noise can be VERY bad, however. The cardboard is basically there to absorb the acoustic vibrations that occur, thus preventing the driveline from making weird or loud noises/hums during specific conditions (such as at a specific rotational speed)."
I imagine the driveshaft could pick up some gear noise from either rear end or tcase/trans and basically amplify it.