Non-Stop Noise from Inner Ceiling of the Microwaveoven

It sounds like the 'stirrer' which is a metal fan-like device with a plastic bearing that is mounted in the wave guide above the cavity. Pull the plug, then the case. Remove the waveguide above the cavity which should expose the stirrer. Spin it and check for any up and down motion--straighten the axle part so that it turns true. The blower system turns the stirrer--no extra motor. If this part was bent, someone may have handled it roughly. Reassemble the waveguide and test without the case on, but a cup of water in the cavity.