Did you try a different pair of XLR cables? It could be something as simple as a lose ground wire inside your cables.
Did you try a different disk player in your system using your same cables to isolate to the Oppo?
Even if you just borrow a dac to help in evaluation, feeding it from the Oppo's digital outs could isolate the Oppo analog board/outs as the source of the noise.
Have you tried physically moving around your cables and messing with any cable intersections, power cables touching interconnects, cables looping, etc. that might be acting as antennae and picking up RFI?
Lastly, if this issue "appeared one day" after having had the unit without trouble beforehand, it certainly sounds like something needing repair more so than you just not liking the presentation of the Oppo.
Modding broken gear is worse than putting lipstick on a pig. IMHO, you need to find the root cause and eliminate the it. Cheers,
Spencer