unhooked or broken motor lead?
The first thing I would check is if one of the three leads to the motor is open, loose, or if the wire from the winding in the motor is broken. I had this happen in a motor after a bad landing and after I reconnected the wire the noise and chattering motor was okay. If you have plugs on the motor, you can disconnect one at a time to see if the winding has a fault. When you unhook the one that is broken nothing will change. Good luck, I hope it is something simple. Marty