I'm very skeptical of online reviews, but had to review this because it's much better than the price might suggest. It really is almost silent, when the speed dial turned lower, which is all I need. Mine is powering a very large sponge filter on a 100L+ tank, and the bubbles popping on the surface are louder than the hum of the pump. There is a slight 'white noise' type sound, which I think is unavoidable on all pumps, as the vibrations travel along the air tube and into the water. This is quieter than the bubbles popping though! I'm only using one outlet currently, so some Bluetack over the unused hole makes a HUGE difference. A good buy!