I decided to power up my Corsair 750TX PSU yesterday for the first time to make sure it worked. I am using it without anything hooked up to it. I jumpered the green wire with a black wire on the ATX connector so I could run the PSU without hooking it up to the mobo.
It works, but it is reasonably noisy IMO. It's not the noise of air...it's sort of a rattling/sputtering rythmic noise which is much louder than the air from the fan. I don't think it is the fan/fan motor. As soon as I turn the PSU off, the fan keeps spinning at almost full speed for a few seconds, but the noise immediately stops.
It actually sounds just like a diesel engine on a really small scale. I have a recording of it, but it doesn't look like there is any way to get an attachment into my post.