That sounds like a bad bearing. Bearings are reasonably cheap. For those small ones just a few dollars each.
Do something sooner rather than later, a siezed bearing can chew the shaft and then you will be up for a new shaft as well as bearings and it could cost you quite a bit.
It could be motor bearings or the bearings on the vibrating shaft. But most likely the vibrating shaft.
Give the shaft a bit of a twirl with your fingers without the belt on, any grinding grabbing feeling in the bearing will indicate the faulty one, they should run smoothly with absolutely no noise when twirled rapidly.