The Rio is pretty quiet. If you do like the water noise you can fill it to right at the level of the pump output and it will give you a bit of noise, but also agitate the surface, though, which may not be the best for CO2. It will probably bug me, but may not bug you. I know of a few LFS's that have it set up in the stores. Maybe there's one close to you that you can listen to. Just make sure you tell everyone in the store to keep quiet for a minute while you conduct a test!
André