That is hard to say from here.
Could be the bull gear assembly or the motor or the motor shaft / pulley.
Does it happen in both high and low?
If only low, I would look at the bull gear set.
Both high and low, I would look at the pulley to motor shaft interface or the spindle.
Here is a link to replacement parts.
Sorry, not much help from 1000 miles away. Sometimes you just need to be there.