From my point of view it is louder than the Enermax Liberty 500 I had in the previous build.
At idle, shouldn't be much difference between the Corsair power supplies, because they use similar fans (all are produced by ADDA, different speeds). And, especially this one, shouldn't be loud at idle/low power systems, because it is a low rpm fan. I find it odd that there is such a big difference between CX400 and HX620 in terms of sound.
In other words, it's my loudest piece, I can hear it, at night, from about 4m away, and this without any load.
The best PS, in terms of sound, I came in contact with, was a "no name" one (Green Power or something), equipped with a Yate Loon , 120mm, low rpm fan. If I remember well, I barely could have hear it from a close distance (10-20 cm); to be more specific, a 12 cm Nexus Real Silent Fan was louder.
The future should be a high efficient PS equipped with a low rpm 14cm fan and heat pipes (like Zalman has) in addition to the classic radiators .
Phenom II X3 720BE :J&W JW-790GX-EXTREME :Geil Ultra 2x2GB DDR2 800 CL4 :Samsung HD322HJ :Nec AD-7203S :Audigy SE @ X-Fi Extreme Audio :Razer Deathadder :Logitech UltraX Premium : Corsair CX400W <<>> Old : Athlon X2 4000+,BOX :ABIT KN9-S :Samsung HD321KJ :Nec AD-7170S alit 7300GT 256MB/128bit :Enermax Liberty 500 :Samsung 795DF Black/Silver :A4tech SWOP-48