I just went though this on my system. For mine, the noise was generated by the video card. I have a usb from the computer to a RME adi-2 dac. I tried ferrite beads everywhere, but no significant improvement. I had it in my mind that the video cables were transmitting this noise, so I got shielded ones.
No point in telling everything I tried. The happy ending came when I remembered that the dac has a toslink/optical socket. I hooked up the optical cable, configured the computer to send sound to the spdif output, and the noise was completely eliminated.
I believe what is happening is that the inside of the computer is filled with the emi emitted by the video card, and it rides on the usb cable to the DAC. I think the dac should have done a better job of rejecting emi, but it is what it is...
I have ordered a gadget that claims to isolate away emi on a usb cable, so I will see how that goes. Thing about the spdif link is that this uses the clock on the computer rather than the clock on the dac, so it may be subject to jitter or clock inaccuracies. It sounds ok to me as is, but that question is floating around.
I wish I had known that the emi was in the usb wires from the computer, as I have had this issue for years over other devices, but only with the rme device did it get loud enough for me to go after it in with my full attention.