Power Supply Issue?

[Calsceron]

Issue: Computer will not boot. No POST, no fans, no hard drives spin, absolutely zero happens when you press the power button.

If you remove the PSU, and short the wires, the fan begins spinning and the PSU makes an odd humming sound. I don't mean the fan, I mean the actual PSU hums. I shorted the power switch and got the same result, so I know the switch is not the issue. Here's the weird part, I connected the PSU to a PSU tester, and all voltages on every rail check out.

What's the issue? I say it's the PSU, but I honestly don't know. Any ideas?

[Zero82z]

A humming noise is not great, but it also doesn't necessarily indicate damage. The obvious thing to do would be to test with another PSU and see if the motherboard will work. In any case, it would be a good idea to replace the PSU, but I would first try and isolate the issue by testing with different components.
Here's the weird part, I connected the PSU to a PSU tester, and all voltages on every rail check out.
PSU testers are useless since the PSU isn't under any load. Voltage regulation depends a lot on how the PSU is loaded, and a no load or cross-load situation will cause it to output voltages which are probably not representative of how they would be in a real-world situation (especially if the PSU is group-regulated).

[Xeth]

Usually not a good idea to run a PSU with absolutely no load attached, they tend to do weird things like hum. Slap on a hard drive and a fan and check voltages again.

[Freezebyte]

Easiest way to tell? Plug in a different PSU.