hmmm. this sounds like my initial condition. mine makes a high pitch whirring sounds, and knocking on it might make the sound go away for some time then it returns.

i fixed mine by washing it cleanly, maybe some debris was lodged inside. you can take it the pump apart from the pump top, it will looks like this http://i.imgur.com/X3Zf5ua.jpg. the middle black color thing is the propeller which is held to the main body with magnetic force and you can easily pull it apart from the main body. once you pull it apart, you can see a stem with a ceramic tip. i washed the whole thing properly, making sure no water touches the PCB at the back and then reassembled everything.