Interference

Does this happen if you open a blank project or even if Sonar is opened without a project at all? (ie, a blank screen, no tracks)

Having opened a new/old project can you stop the hum by muting either the Master bus,  or the Interface Outputs?

In that case, you're saying the hum is starting at a point after Sonar has taken control of the driver (if that's the correct term) but before the audio engine is initialised.
 
If it's mains hum, my first option would be to check every single cable in the system. Not just on the audio interface.  Especially where wall warts are used.
Can you get 'silent' playback from Windows Media Player?  Do you have any loop back/through? Have you got input monitor switched on with the input gain turned up high?  Or a guitar lead plugged in but not connected to anything else?
 
Does the "hum" go away when you load a new/old project and mute the Master or Interface Outputs?
 
Try all these things when you get home.
 
 

Try opening any graphic-heavy application (a game or heavy graphic web page) and listen if you hear the hum.
 
Btw, is it a low-frequency or high-frequency hum, more of "buzz" ? :)
 
I have had interference going on lately - when I was moving with mouse or my HDDs were under heavy load or my graphic card was under load - strange hum/buzz was coming out of my monitors.
 
I solved it by using balanced XLR cables between my audio and monitors. I needed an adapter XLR -> TRS because Scarlett audio card does not have XLR outputs...

Thanks to everyone, only getting back now, bought new leads (balanced) and changed them last night--  woop woop no more hum!
 
Thanks
 
Nigel