Hmm...
I see that you're using the on-board sound card.

Check inside the case and make sure nothing is coming in contact with the motherboard, particularly around the audio chip/codec or the jack cluster. You may even want to remove the motherboard from the case (yes, I know, a real pain) and run it on a non-conductive surface to see if that helps.
Also, if you have one handy, I'd try running a separate PCI sound card and disabling the on-board stuff.