When I use the convection setting the motor runs but makes a noise. Not sure if it is the blade mounted on the motor that is hitting the cover or if the bearing on the motor has gone bad. How do I determine which one is it? If it is the motor, I would like to have the instructions. I don't want to take the oven out of the wall to install the motor is my hope. Any one has service manual that I can see the replacement procedure.