I am making progress! I have discovered that the problem is ONLY pesent when I have audio cables plugged into the Kurzweil (and octa, of course). Interestingly (to me) the Kurweil does not even have to be powered on for noises to appear at the monitors. If I unplug the audio cables from the Kurz - SILENCE! - even if I have Dim Pro running and using my Remote SL for midi input. Therefore, I have to conclude that the audio cables from the Kurz are acting like antennae for RF interference. I have tried every audio cable in my arsenal (nearly all are unbalanced but there was one short pair of balanced cables). Some are better than others, none are perfect. The balanced cables I thought might be the solution but they are no better than average. That said, they are probably pretty cheap balanced cables. I really don't even know what I ever bought them for. So, unless someone has a better suggestion, my next step will be to get some decent balanced cables and see what happens! Once again, thanks for all of the suggestions!

Try one of these http://www.amazon.co.uk/gp/product/B000NVWB9O/ref=oh_details_o00_s00_i00 I just installed it betveen my Alesis I/O26 and my mixer, all noise gone including mouse movement noises, and a annoying high pitched tone.

Hope this helps,

You dont have any of the cables routed next to power cables or the monitors do you?

The only other thing Ive seen that hasnt been mentioned is a similar prob noted with dual monitors, one monitor had developed a prob causing a similar noise prob and switching to one monitor and swapping monitors and cables proved to find which one was bad creating the noises you describe.

I will look for one of those ground loop isolators. That will be a cheap fix if it works. Funny you mention the annoying high pitch tone because I did notice that after swapping some cables around. I figured it was the cables cuz only happens when certain cables are used.
I do have some audio and power cables which cross each other. I try to keep them at 90 degrees to one another to minimize interference. I have taken my system apart this week and am reworking the layout so I may be able to minimize interference with better cable management.

Time to summarize. Over the past 10 or 12 days I have totally taken my system apart and reassembled. I have looked inside the computer and found a massive tangle of wires but nothing I can really do anything about because it is a very tight mini-tower case. I have changed over to all balanced cables for audio. I have removed the Octa from the same cabinet that holds the PC. I have paid attention to cable nmanagement and minimized power and audio cables in proximity to one another. And STILL I have noise. As mentioned earlier, grounding the Octa to the PC did help somewhat. Also unplugging audio outs from the Kurz silences everything but iis obviously not a great solution. Also, grounding the Octa to the Kurz chassis does nothing BUT grounding it to the metal on the outside of the balanced audio cables at the Kurz outputs results in SILENCE! (better than grounding to PC). So, maybe it's not rf picked up by the cables but some sort of grounding issue between the Kurz and the Octa?? (Obviously I am ot an electrical engineer!). For now, I am grounding the Octa to the audio cables at the Kurz which sounds good but looks weird! I remain open to other, better solutions!