In case someone should find this useful…
It’s common knowledge that the power supply on a computer is usually the source of any strange ringing sound. However, one usually expects that the ring would appear under load – i.e. increased power = increased ring.
I just found a PSU that doesn’t ring under a small amount of load, using the integrated video card. When I installed a video card, it rang as long as the card was idle. When I put the card under load, it stopped. I thought it was the video card or the PCI-e interface. Guess what! It was the PSU. It only rang in a small power draw range.
The moral of the story: If you have a ringing computer, it’s probably the power supply, no matter how irrational the observed behavior!
Update: Replacing the power supply seems to have fixed all of my problems with JACK xruns, too – go figure. (if you don’t know what that means, just ignore it…)