Diagnosis
The rotisserie motor is jammed or its gear train is stripped. Often caused by an overloaded spit or food debris.
How to Fix It
Unplug. Remove the spit and rotisserie basket. Manually try to turn the silver drive socket inside the right wall. If it doesn't turn freely, debris is jamming it. Clean it out with a small brush or compressed air.
If That Didn't Work
If the socket turns freely but the motor still hums when powered, the plastic gears inside the motor housing are stripped. The motor assembly needs replacement.
Heads Up
Never force the spit into the socket. Ensure it's properly aligned and seated. An unbalanced load will strain and break the motor gears.