DIAGNOSIS The drive coupler—that's the little three-pronged plastic piece under the glass tray that connects the motor to the tray—is either broken, worn out, or the motor shaft itself is obstructed. Food debris can also jam the mechanism.
FIX 1) Unplug the microwave. 2) Remove the glass tray and the plastic ring guide underneath it. 3) Look down into the center hole. You'll see a small, white plastic coupler with three arms. Check if any of the arms are snapped off or if there's crud packed in there. 4) Clean out any debris with a dry cloth. 5) If the coupler looks intact, gently try to rotate it with your fingers. It should turn smoothly and engage with the motor shaft below. If it's broken, you can order a replacement coupler (part number is often something generic like WB17X10026) and just pop the new one in.
IF THAT DIDN'T WORK If the coupler is fine and turns freely, the issue is likely the turntable motor itself. You can feel if the motor shaft is trying to spin when you run the microwave (carefully, with the tray removed). If the motor hums but the shaft doesn't turn, the motor is seized and needs replacement.
HEADS UP Always make sure the glass tray and ring guide are seated perfectly. If they're off-center, they can bind and cause this same symptom and noise.