Diagnosis
A foreign object has entered the fan housing at the rear of the appliance. Common culprits are small pieces of parchment paper, a loose piece of food, or a broken piece of the basket coating.
How to Fix It
Unplug the appliance. Let it cool. Tilt the unit forward and gently tap the back to dislodge the debris. Use a can of compressed air to blow into the rear air intake vents. You can also use long-nose pliers to carefully pull out any visible debris through the vents.
If That Didn't Work
The fan impeller itself may have cracked or come loose from the motor shaft. This requires removing the rear cover to inspect. A broken fan will cause overheating and error codes.
Heads Up
Never operate the appliance with this noise. The fan is critical for moving hot air. A blocked or broken fan will cause immediate overheating and may trip the thermal safety fuse, requiring a full board replacement.