I was listening to my system up close in a quiet room yesterday. I have all the fans installed now (3x Scythe PWM + Mugen w/PWM) and I could hear a soft, helicopter-like, whirring/pulsation.
I couldn't quite localize the noise with the case in its position near the floor, so not sure if it is the PSU fan or others. Not really that noticeable from where I sit, but I will listen again next week to try to isolate the noise. (Once I've got the computer back up on my worktable to install the spinning hard drives, I should have an easier time locating the source.)