in the audio preferences...with my StudioCat computer... Unchecking this makes my active monitors MUCH quieter.  With the track view as the top window...I get low level nasty-ness...  like RFI sounding stuff.   Open ANYTHING on top of that window...noise goes away.
hhhhmmm   wonder what that's all about.
 
HELP files don't say much about this... kinda a simple sentence half describing what it does.
 
btw...I re-vive an old thread in the "hardware" forum...  but it's too quiet over there... like my computer now!!!

If you have Share Drivers enabled in SONAR, changing focus away from SONAR will close audio ports, silencing any noise. That's normal, but the noise is not.
 
And you don't want to be disabling the multi-processing engine; that's what let's SONAR use all cores on your CPU to balance the load.
 
It sounds like you have a grounding issue or interference from mouse, video or some peripheral. You should take it up with Jim Roseberry.