Hmmmm... so after reading the post about hum from PRO CHANNEL I thought I might post my observation of a different problem but similar.
 
When SONAR is open I hear computer glitches coming from my computer.  Like HD swirls etc.. just basically high pitch noises.  I thought that this might be coming from my monitors being too close to the display screen so I went out and purchased TRS shielded monitor cables thinking that might fix the problem... no luck. So, the last few sessions, I have noticed something real strange, the second that SONAR isn't the active task, the noise goes away.  That is, open the UAD monitor (or any other app like TASK MANAGER) on the same screen as SONAR, noiseless, as soon as I give SONAR (SPLAT) focus, the noise comes back. Realize that the SPLAT screen is still visible, just covered up a little by the small UAD monitor rectangle.  This little observation has ruled out the graphics card as the culprit.
 
Any insight into this?   I don't think the audio is being recorded but it is very annoying.
 
Thanks,
Jim
 

The reason the noise goes away is likely that you have SONAR set to Share Drivers in preferences so its closing the ports and letting go of the interface drivers when it loses focus.
 
Hardware activity causing noise in the monitors sounds like a grounding issue inside the machine or the interface. You might want to start with Mr Roseberry to troubleshoot that.

Mr Brundlefly,
 
Thank you for that input.  I do, in fact, have Sonar set to share drivers.  So, in theory, the interface is external (UAD Apollo) so that if the noise was sourced at the interface, then it should continue after SONAR loses focus, no? So, if I am playing a YOUTUBE video or some other audio software, shouldn't the noise return if it was the interface or a grounding issue?
 
Just wondering? and thanks again for the insight.
 
Jim

Probably electromagnetic radiation from your computer triggered by your USB or other external devices, perhaps your audio interface. I have exactly the same problem and it's driving me mad because it gets picked up by my guitar and recorded when I track. I have a thread about it here - http://forum.cakewalk.com/EMI-from-USB-devices-being-picked-up-by-guitars-m3363937.aspx - and another about it here - https://www.gearslutz.com/board/so-many-guitars-so-little-time/1065324-how-stop-1-khz-hum-computer-being-picked-up-guitar-pickups.html - but there are so many different causes that the advice people give is often not very useful.