HF noise from a switching power supply will appear as a broadband noise centered at regular intervals. These intervals will commonly be around 70 kHz. If the noise is broadband without peaks at regular intervals it may be the PC, printer, ethernet, and so on.
Clamp-on ferrite beads are often the first thing to try. You might first narrow down the problem to the worst offending equipment. RFI is like an onion, as one layer is removed another will be revealed. To do this power off the computer equipment one at a time to find the one with the highest RFI. Now apply ferrite cores to the leads. You might try routing all the leads together through cores. DX Engineering sells suitable cores. http://www.dxengineering.com/search/part-type/rf-suppression-snap-on-ferrite-beads?tw=ferr&sw=RF%20Suppression%20Snap-On%20Ferrite%20Beads