Diagnosis
The convection fan blade is hitting something. Usually it's a piece of dislodged carbonized food or a warped drip tray.
How to Fix It
Unplug the oven. Remove the wire racks and drip tray. Look at the fan at the back of the cavity. Use a flashlight to spot debris. Use long tweezers or needle-nose pliers to remove any lodged food. Check that the rear fan guard isn't bent inward.
If That Didn't Work
The fan motor shaft bearings are likely failing. The motor is a sealed unit attached to the rear wall. It requires disassembly of the entire outer shell to access and replace.
Heads Up
Do not run it with the grinding noise. It can shear the fan blade off or damage the motor windings, causing a more expensive repair.