I have a used one, it is quite for a day or so then seems to move slightly, the more it moves the louder it gets from a range of hums to a jack hammeresque sound. Its on a 45g tank but the water movement is great and can be tuned out by turning up the TV or adjusting its placement.
Edit,
The humming is not that loud as i just read that if the bearings are bad I would hear it from downstairs. Also my cat might be moving the cord on the dry side making it off balance to make the noise. Damn cat. I like my MP40 but if it wasn't given to me I wouldn't spend that much cash.