at high speed the blower will make noise and slow down. if you lower the speed
it works fine
The motor of the blower is connected to a set of veins which spin inside the housing. They are made of plastic. If the veins become loose from wear where they connect to the shaft, they will begin to vibrate. Remove the blower motor and check the movement of the veins. If you need a replacement, bring the old one along. As sometimes replacement motors have their veins rotating in the operation direction.