DIAGNOSIS The blower wheel has likely come loose on the motor shaft or the bearings in the motor are failing. Debris can also cause imbalance.
FIX Unplug the unit. Remove the vent grille and the charcoal filter. Inspect the blower wheel. Tighten the setscrew on the wheel hub if it's loose. Remove any visible debris.
IF THAT DIDN'T WORK The vent motor assembly itself has failed. You'll need to replace the entire motor, part WR55X10934. This requires removing the outer wrapper of the microwave.
HEADS UP Do not run the vent with a grinding noise for long. It can seize and damage the control board. Discontinue use until repaired.