Using caution apply a small amount of pressure to the CPU fan, obviously don't stick your fingers in the blades, but if it has a solid middle apply pressure there, if it doesn't squeeze the sides, you only need to do it enough to alter the fan speed, do NOT stop it, it's just in my experience a bit of pressure on the fan removes or highlights if that fan is being noisy.
Once you can eliminate that fan and your sure everything else is fine it just leaves the PSU, do you have a spare you can try to check ?
If your confident about computers unplug the power leads going to the board and see if it will power up, obviously the system wont, but I'm hoping the PSU will, if it does and the noise is there then you have your culprit, but I'm not to sure if this will work as it's been a while since I messed around like that.
But you can get electrical whines as well, but if you isolate it to the PSU it's probably time to get a new one, opening a PSU is dangerous and can kill you if you touch the wrong thing, there are components in them that contain 1000's of volts, so only qualified electricians are advised to go into such things, hope this helps.