Diagnosis
A foreign object is hitting the fan blades, or the fan motor bearings are failing.
How to Fix It
Unplug the unit. Look into the top heating chamber with a flashlight. Use long tweezers or needle-nose pliers to remove any visible debris like a piece of bone, hard bread crust, or foil. Check that the fan blades are not bent.
If That Didn't Work
If no debris is found, the noise is internal. The fan motor shaft or bearings are worn out. The fan assembly is mounted to the main housing and requires full disassembly to replace.
Heads Up
Do not operate the unit with a grinding noise. A failing fan motor can seize, causing the unit to overheat and trip the thermal fuse. It can also be a fire risk.