DIAGNOSIS That's the convection fan blade hitting something, or its motor shaft is out of alignment. The most common culprit is a small piece of food debris (like a crumb or seed) that got past the guard.
FIX Unplug the oven. Remove the racks and tray. Look at the fan assembly on the back wall. Use a flashlight and a long, thin tool like a wooden skewer or plastic tweezers to gently probe around the fan blade and its central hub, dislodging any debris. Manually spin the blade to see if it moves freely.
IF THAT DIDN'T WORK The fan blade itself can work loose on its motor shaft over time. If you can safely access it (consulting a disassembly guide), check if the blade is secure. A persistent grind often means the motor bearings are wearing out.
HEADS UP Never use compressed air to blast the fan while it's in the unit—you can force debris deeper into the motor. And never try to stop or manually spin the fan while the oven is plugged in or running.