DIAGNOSIS: Debris has entered the fan housing or a fan blade is bent and contacting the shroud. Common cause is food or oil particles slipping past the basket.
FIX: Unplug and let the unit cool completely. Remove basket and pan. Tilt the main unit upside down over a trash can. Use a can of compressed air to blow forcefully into the lower vent openings. Rotate the unit and blow from different angles. Do not insert tools. Reassemble and test. Noise should be gone.
IF THAT DIDN'T WORK: The fan motor bearings are failing. The fan assembly is not designed for user replacement. This is a warranty/repair issue.
HEADS UP: Always let the unit cool and drain grease from the pan before storage. Oil pooling at the bottom migrates into the fan area when hot.