DIAGNOSIS The noise is from the cooling fan mounted on the rear panel. A loud hum indicates an obstruction or a failing fan bearing.
FIX Clear any obstruction. 1. Unplug the unit and let it cool completely. 2. Inspect the rear fan grill for any visible debris (lint, pet hair). 3. Use a vacuum hose attachment or compressed air to clean the grill. 4. Ensure the unit has at least 5 inches of clearance from the wall for proper airflow.
IF THAT DIDN'T WORK If the hum persists after cleaning and with proper clearance, the fan motor is likely defective. The fan is integral to the rear access panel. This requires replacement via Ninja customer service.
HEADS UP Never operate the air fryer with the rear panel removed. The fan is necessary to prevent overheating of internal electronics.