Chances are the rotating parts are fairly seriously out of balance. Does it have a blade plate or just a long, single blade? If it has a blade plate, try removing the blades and their attaching hardware completely and see if the balance (that is, vibration level) improves. If it does, replace all the blades and their attaching hardware with a new, full set. If it has a long single blade, remove it and set it horizontally on the shaft of a screwdriver held horizontal. If one end drops lower than the other, grind back the sharpened cutting edge at the end of the heavy end until it balances.
