More likely to be the element around the fan has buckled and bent and is now touching the fan blade. Take the cover off & look. If element is bent then replace the element. Don't go bending the fan blades as a temporary fix. Do the job once & do it properly.
Lincoln Appliance NZ
December 2016
Fan blades in cookers are usually made from aluminium - with the passage of time the constant heating and cooling can cause one of the blades to bend - what you hear is very probably a bent blade going clang clang against a near object.
If the fan is on the back wall of the oven then remove the cover plate and adjust the bent blade.
Or - if you're short of confidence - call in an engineer or a friend.
Good Luck...
December 2016
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