The pump is damaged or there is heavy clogging within that prevents the motor to work.
If there is wearing of the pump bearings or bushes then the rotors might get struck resulting in the hum. Secondly check the CAPACITOR connected to the motor , if the capacitor is tested and shows low value then you can replace.- simple test is to charge on the mains and see if there is retaining charge to prove it works. If not replace.
Finally remove and clean the motor and pump assembly.