This is usually caused by the element being distorted, remove back panel and check if element is out of shape and catching fan blade. Could also be a hard grease deposit on back of backplate catching fan blade. If oven is double built under the counter the element is half round shape, on this oven the top screw holding the element behind the fan blade can come loose and catch the fan. Endfloat on the fan shaft is not a prob. NOTE if removing fan blade its nut is LEFT HAND THREAD! Cheers
Kenjo
May 2012