Quick questions about ATX power supply

[egasimus]

I've got an old PC power supply, which is supposedly able to dish out 10A at +12V, and 0.5A at -12V, which ought to be plenty for small-scale development and testing purposes. I tried to turn it into a bench PSU, then abandoned it for a while. Now that intend I to build a thing or two in the way of modular synths, I think I'm gonna turn to that old beast again.

However, it might need some reconditioning, and this is where I need some help. A while ago, I powered a small power amp from its +12V rail, and, surprisingly, it hummed - unless I touched the PSU's chassis. Grounding seemed to be sound, so I was at a loss what caused it - aren't those PSUs supposed to be very well regulated?

Also, I did at one point short its +12V and +5V outputs. It didn't have an immediate effect, but I just don't trust that PSU any more. Is there anything I can do without an oscilloscope to check if it's still sound and reliable?