I don't think any air pump is going to be "silent" but you can get some that are much quiter than others. From my expierence with an air pump on an air stone the noise seems to come from the resonance of the air in the tank against the glass if that makes any sense. Basically the pump isn't the problem it's the force of the air coming out of the stone in the tank that is amplified throught the glass if it is attached. It seems that if it is free floating it isn't loud, but once attached to the glass via diction cups the noise is quite loud, which is why I run my stone during the day while I'm gone and turn it off at night.