DIAGNOSIS The convection fan blade is hitting something. Food debris or a broken piece of the heating element shield has likely fallen onto it.
FIX Unplug the unit and let it cool completely. Remove the cooking pot. Look up at the fan assembly at the top rear of the cavity. Use a flashlight and long tweezers or needle-nose pliers to carefully remove any visible debris from the fan blades and the area around them.
IF THAT DIDN'T WORK The fan motor shaft bearings have worn out. This requires disassembly and replacement of the entire fan motor assembly.
HEADS UP Do not try to spin the fan manually with force. You can misalign the blades and make the noise worse.