A microwave equipped with a turntable has a drive motor in the center of the oven floor that spins the turntable. The turntable is made of glass and has grooves on the bottom of it which seat in the internal grooves of the drive-motor bushing. The outer edges of the glass turntable ride on a turntable guide equipped with wheels. These wheels roll in a groove in the bottom of the oven floor. When food particles gum up the wheels of the guide, squeaking occurs.
Unplug the oven.
Remove the glass turntable from the oven and wash it with dish soap and water using a scrub brush.
Remove the turntable guide from the oven and soak it in hot soapy water for 15 minutes. Scrub the wheels with the scrub brush until cleaned of all food debris.
Wipe the oven floor of the microwave with a soapy rag to remove all food debris. Allow the turntable, the guide and the microwave floor to dry.
Replace the guide and glass turntable. Plug the microwave in. The next time you use it, listen for squeaking. If squeaking persists, seek professional help.