I just use the regular fan that comes with the psu and it works fine, and there's not much noise either. Do you have a special kind of PSU that needs serious cooling? If you have an old junk PC you can salvage the fan from it and lube it up with grease or oil (or a mixture of both) to make it quieter. Another thing you can try is putting in a smaller fan, for example use a cpu cooling fan, only concern is if it does not cool the thing enough. I had to do that with a 24 port 3COM switch. The thing sounded like a lawn mower even after lubercating the fans. It uses two high output fans which I replaced with two identical cpu cooling fans. So far it's been working for a month and makes almost no noise. Hope it lasts in the summer heat.
You could also try building a bigger box for you rmachine so it gets fully closed up, or build it using thick styrofoam as noise insulation instead of wood, or why not just buy a real case for it?
One final thing you can try, is suspending the fan in air using string or thread instead of having it attached to the case with screws, that'll prevent noise from vibrations. You can test it out by holding the fan while its running with your fingers and see if it quiets down or not.
.. or buy one of those silent fans
Good luck!