to the best of my knowledge there is no such thing as a fluval 406 unless it's VERY new.. i have several 405's however and they are VERY quiet. i had a fluval 405 on my 110 gallon in my bedroom for 3 years, only time i heard a sound was when the evaporation brought the level down to the point that i got waterfall noises.. i only had a couple fish in the tank, so i waited till it needed to be refilled to do water changes, never had more than 10 ppm nitrates.
i'm a big fluval fan, i like the 405s and i REALLY like the FX5.