Easy to repair, switch off the power to the oven, just undo the four screws that holds the cooker in the kitchen unit, slide it out, careful you dont drop the oven, when it is out remove the back plate, six screws,
now you will see the fan motor, put one drop of oil on the end of the bearing, its the bit that sticks out the futherest, then rebuild, and your oven will run smooth again.
Sandy, May 2010