I solved my pc noise issues after I realized it was usb that was the culprit (for my setup). Breaking the usb earth loop, isolating the usb power (from pc usb port) and for powered usb devices, using external low noise linear 5V power supplies did the job for me.
There are some tools out there.
Thanks for this post. I've done some reading and some asking, but mostly get pretty broad or vague suggestions (try shielded cable, etc). This sounds much more like what I'm dealing with. I'm going to be building a new PC soon, is it ok to hit you up then and pick your brain just a little on the topic?