Diagnosis
That high-pitched squeak is almost always friction between the plastic turntable roller ring (the track on the bottom of the glass plate) and the three plastic support wheels inside the microwave cavity.
How to Fix It
You need to clean and lubricate the moving parts. 1) Remove the glass turntable and the plastic roller ring underneath it. Wash both in warm soapy water, drying thoroughly. 2) Wipe the three small plastic support wheels inside the microwave floor with a damp cloth to remove any grease or food grit. 3) Apply a tiny, tiny drop of food-safe silicone lubricant (like for kitchen appliances) to the groove in each support wheel. Do not use cooking oil—it will gum up. 4) Reassemble. The squeak should be gone.
If That Didn't Work
If the squeak persists, inspect the roller ring for cracks or deep wear grooves. Also, check that the glass plate is sitting perfectly flat and isn't cracked. A warped or damaged ring or plate can cause uneven pressure and noise.
Heads Up
Make sure the turntable and ring are seated correctly. If the ring is off its track or the plate is off-center, it can bind and squeak. The arrows on the plate and the hub should line up.