The noise associated with belt problems has mostly to do with flopping of the belt. It is visably noticable to anyone who has seen it. All I can say is that if the problem is with the belt it should be visable.
Motor mount problems are not so distinct. I would guess that all problems of a motor mount nature would be easily detectable
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician