There is a big chance what you are hearing has nothing to do with over speed and has everything to do with the motor vibrating violently due to the mounting mechanism breaking. The motor on this unit is fastened to the housing in a cantilever style which means if the unit is dropped or hit hard, the weight of the motor creates a torque that can break the plastic at the mounting screw. This is due to bad design. I bought two of these, opened them both and they both broke at the exact to point; the thin plastic that secures the motor to the housing.