Diagnosis
This is a foreign object obstruction or a misaligned fan blade. Debris from packaging or initial use has likely entered the convection fan housing at the top rear of the oven cavity.
How to Fix It
Unplug the oven. Remove the two screws securing the metal convection fan cover at the top rear of the cavity. Lift off the cover. Manually rotate the fan blade and inspect for any plastic, tape, or food debris. Remove all obstructions. Visually confirm the fan blade is not warped and spins freely.
If That Didn't Work
If the noise persists with no obstruction, the fan motor bearings have failed. The convection fan motor assembly must be replaced. This requires disassembling the outer top cabinet of the oven.
Heads Up
Never operate the convection functions with this noise present. It can cause the fan motor to overheat and fail completely, potentially tripping a thermal fuse.