I would start out with the easier stuff first. Make sure the mechanical parts are all tight. Check the motor mount, any rod connectors or pulleys. Check belt tension. Check for any pulley or connector wobble.
Once that's all cleared, then start into the electronics. Check wiring is properly connected per the pinouts. I would remove the offending motor, lock the coils and try the shaft with a pair of locking pliers. Check for the shaft moving in or out of the motor, or twisting.
That should at least get you started.
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