The fan in the Corsair 400CX is definitely not the quietest in the 120mm PSU realm, but far from the worst. The Corsair HX models, and the Seasonic S12-II's are a bit quieter, though still not quiet enough for me. I've always done a fan swap to an S-Flex "F" in my Corsair and Seasonic PSU's. Since they use traditional voltage van speed control, rather than PWM, it works perfectly.
The Seasonic X650 is silent (no fan spinning) unless your system draws over 200-250w continuously, and the fan is very quiet even when it kicks in. This, and its' 750w big brother, are the best PSU's on the market right now for those of us who want quiet and efficient, but it'll cost ya, about $160.