The tricky thing is, any PSU (aside from low power passive models) is going to get hot under load....
So the fan is going to have to spin up to cool it if its working hard. Im guessing if your water cooled, then air flow inside the case is not great, which in turn will force the PSU fan to work that bit harder.
Im not saying it will help, but a friend of mine has turned his PSU upside down, and cut a 120mm intake hole in the top of his case, so the at least the PSU is drawing ambient air rather than air from inside the case, this should help keeping the fan from spinning up as much (in theory assuming the room is not to warm)
Either that or add a large low RPM case fan just for the cpu to help it breath a bit better and hopefully stop it spinning up so much.
The corsairs are pretty quiet as far as psu's go, but under load, I think pretty much any PSU will make some noise.
