Except I'm using an external card in my BR800.
I don't think this sound was always there (during sound events). 
I now have better monitors (Rokit 6's) and perhaps they have made obvious the sound that was always there?
Your post does make me wonder if my on-board card does the same thing.

Update -Today, at this moment, my tone is not there unless I pull the volume fader right down.
The tone reminds me of the old radio beacon sounds of an ADF giving you the dot-dashes of the station when you get the frequency right. (Ex pilot)

HELLO ALL;
I just wanted to add some input, I am running Cakewalk X3 and have had this issue of noise interference for a while.  I am running a Mobile pre USB box with my Windows 7 Cyberpower gaming PC.   I always figured it had something to do with my settings.   I also hear the noise when I open fruity loops as well.  So the fix was to open control panel and I found a program called VIA HD Audio Deck.  In it I could control my pack panel and front inputs, so I took a hunch and clicked on expert mode and BAM.  Noise gone.  It muted the computer picked default device and in return let my Mobile pre do its job.  OMG the sound is incredible, Im like a kid in the candy store.  I hope this helps!!!!!
 
Lonza "Zay" Ivey 
Ivey League Entertainment
zay2001@gmail.com
 
Sonar X3 Producer
Win 7 intel Core2 Duo CPU 64 Bit Atheros AR8131 PCI E Gigabit @ 2.4 GHz, 4 GHz RAM  
Mobile Pre-USB
Avid Artist Mix, Avid Artist control controller , Axiom Pro 49 keyboard
BX5a M-Audio Monitors" Behringher ROH5 Monitors
Tube MpStudio
2 Furman Power Grids
Behringer Autocom Compressor ProXL
Samsung 50 inch TV-(mixing console)
MXL 770 Condensor mic
IPAD/22 inch MAC 

zay2001
So the fix was to open control panel and I found a program called VIA HD Audio Deck.  In it I could control my pack panel and front inputs, so I took a hunch and clicked on expert mode and BAM.  Noise gone. 

 
Could this thread be another example of the dreaded HD Audio driver issue?
 
Also, FWIW last time I had a high-pitched whine in a laptop it's because the internal mic wasn't muted. Yes, I felt very stupid.

oh there was a typo tho  "control my back panel and front inputs"....  sorry