If the PSU is fanless, then what you need to do is:
Unplug the PC(PSU) from the wall, and hold down the power switch to discharge the PSU properly.
Unplug every device that draws power from the PSU (yes, that includes your motherboard)
Plug the PSU back into the wall socket, and make sure it's switched on. Check for noise.
Unplug PSU, plug in motherboard only, discharge PSU, then plug the PSU back into the wall and check for noise. If no noise, plug the fans back in and then check for noise. If no noise, try to boot system. Assuming it boots, shut it down and check for residual noise.
If still no residual noise, plug in your drives one by one and check them for noise before and after a successful boot (you don't have to get as far as the OS booting, you just need it to POST and to power up everything.
After doing all that, you should be able to isolate the cause of the noise.