Unbalanced fan inside Kitchenaid oven?

I have Kitchenaid oven model KGSA906. The internal cooling fan(?) is apparently unbalanced. Sometimes, but not always, it vibrates when running and makes a VERY loud noise. Other times it runs quietly. If I bang the oven door shut, that might make it stop the loud vibration. A few other tricks can make it sop (e.g., moving the burner grill on the range.) have had three different repairmen look at it, they don't seem to get the problem, they just "tighten things up", which accomplishes nothing. Anyone have any insight into this issue? Suggestions of how to communicate to a repairman to get it fixed?