Yeah I have 3 of them, you might want to open it up and check and see if anything has worked it's way in there, might be a little bit of gravel or something, how long have you had it?
its a grow out tank so no gravel etc in there. i cleaned it out about 30 mins ago. judt checked 5 mins ago and nothing 'loose' or placed wrong.
i seem to remember it buzzing when i got it but not this load (over the time its got filled up with rubbish from the tank and slowed/quitend down abit)