Hi,
I have an older magnum, it is almost 2 years old now and finally the motor is starting to go out. Not too bad, 2 years of continuous service. The noise is probably the impellor cavetating ( sp.) Take the inside apart and clean the bell housing of the impellor and the slot the magnetic potion goes into. Are you also getting bubbles? If so that I am for sure that's what is happening.
HTH
Josh