I dont find it loud at all. Because the pump is in the water, that muffles the pump noise, and all you really hear is the water and air mixture, Im quite satisfied with the noise level too, I had a multiskimmer, and it was VERY LOUD! because the motor was in the housing itself, so it was extremely loud as well as the mixing chamber was in the unit for the water and air.
Like I say, Im satisfied with the seaclone. It may not be the most effective (as some people say), but it's doing a great job for me and my tank.