In my experience of using air pumps, all them produce noise of both vibrating of pump body and noise of bubbling air. I have been through numerous air pump types and have tried many methods to keep them quiet on my tank in the lounge.
Best of all is to put it on a timer, so it is not on when you are in the room. However the inductive load of most pumps seems to destroy electronic timers, eventually, especially the cheaper time switches (3 for £10 from B&Q). The time switches start suffering from forgetting their settings and just eventually just fail. I used an Ikea mechanical timer for a while (£3, the one you pull tabs out to set time). Not very accurate and did drift, but worked 100% on inductive load. Finally got an expensive brand £20 from electrical wholesaler (Timeguard brand,
http://www.timeguard.com/, with 3 year guarantee), which worked 100%. Now using remote controlled mains sockets from programmable remote.
As for shutting the pump noise up, I tried suspending the pump from string, but if anything made the noise worse as it passed the vibrations into the tank cabinet. A good attempt was made by standing the pump on block of polystyrene, even better was encasing the pump in a box of polystyrene, though got worried about heat. Best noise reduction was cutting a piece of paving stone to fit in bottom of cabinet and putting the pump on that.
Unfortunately since going high tech, the paving stone has gone and air pump is now wedged in cabinet, it's legs just visible above the CO2 plug in the picture below.
