If insulation comes loose from the inside of the evaporator, it can get into the fan cage (the fan is a "squirrel cage" type).  This can cause an incredible vibration, and the excess load usually blows the transistor in the blower controller and makes the blower run only on high speed. 

Sometimes insulation can be dislodged from the fan by whacking the blower motor box with your hand (you have to remove the aquarium cover and perhaps the false bulkhead).  Often though, you have to remove the blower motor.