DIAGNOSIS That's the convection fan motor bearing failing or the fan blade hitting something.
FIX Unplug and cool the unit. Remove the back panel. Locate the large, round convection fan at the top rear of the cavity. Manually spin the fan blade. Listen for grinding and check for any debris (like a piece of food packaging) stuck between the blade and the housing. Remove any debris. If the blade spins freely but the bearing is noisy, the fan motor assembly needs replacement.
IF THAT DIDN'T WORK If the noise persists after clearing debris, the fan blade itself may be warped or the shaft is bent. This requires replacing the fan blade and motor as an assembly.
HEADS UP Never operate the oven with this noise. A failing bearing can seize, causing the motor to overheat and potentially damage the main board.