When you switch the coils on it generates a sudden torque pulse on the motor. The rotor is given a sudden push, and the pump frame experiences the same torque pulse in the opposite direction. Sort of like hitting it with a hammer at 50HZ to make it go around . Something similar happens when the coils suddenly turn off.
You can modify the abruptness of the torque pulses by changing where in the rotation you turn the coils on and off. If you turn the coils on when the magnetic field is at it's strongest on the coils you will get the worst case, if you turn the coils on before the rotor rotates into the strongest part of the field the initial torque pulse is less.
You can also reduce the abruptness of the torque pulse by turning the coils on and off slowly which is essentially what a sinusoidal drive does.
You can also get noise from the metal laminations that are used to build the core of the electromagnets. As the magnetic field is turned on and off they are alternately pulled together and apart. This is not usually a dominate noise source.
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