Digital noise

Sound very much like a sampling rate or bit-depth issue. Double-check your clocking, obviously to match your session bit-depth and sampling rate. Sometimes Sonar doesn't display this as you'd expect. Your driver or Sonar may attempt to sample or playback audio at whatever bit-depth it can, whether it's right or not.

If your interface is trying to transmit the audio at a lower-bit depth, such as 16-Bit, this is obviously bad for you.

Check clock source (maybe try reversing the clock source to the rig or to the FireBox), check bit-depth and sampling rates of everything you have in the digital signal chain (DAW, FireBox Control Panel, drivers, etc.). It's in there somewhere. Trial and error may be necessary. Try a different session as well. I found it to be merely the Waves L3-16 limiter I was using causing this, and I still don't know why.  It could be a plug-in.  

Good luck!!

Hrm, well when I change sampling rates there's a noticeable change in the TYPE of noise, but still the same family of noise.

Tried switching bit depths and sample rates and it's still there though. The Firebox doesn't seem to have a discrete bit-depth setting so I wonder if it's converting on the fly? I'm using 24/44.1 in SONAR and have the Firebox set to 44.1 as well.

The only option for clock in the Firebox is internal or SPDIF, so I don't know what options there are to "reverse" it (by which I think you mean to have SONAR generate and Firebox lock to it?) 

On the electrical side I tried swapping firewire cables out as well, I've got three and none of them make a difference. I did a manual inspection of connections and they all look good, and I tried swapping my video card to a different slot to no effect. I don't know what else to on this end except for trying to meter each connection for some kind of drop?

^ good question from Guitarhacker

also, did this just start?  was it working fine earlier?  did something change when it started?

possibilities:
your computer power supply
your computer video card
your compuer monitor (try disconnecting it)

I don't think problem just started, but now that I'm in a new physical space I seem to be more attuned to it or it is louder than before. Well shoot, I guess in a way then yes, the problem has just started.

I did just rebuild my rig to Win7/64

I did check the presonus site, I can dig deeper but most of what I found there was all about ground lifting.

I need to get my hands on a different audio interface for testing. I'll see if I can find a generous soul. :-)

A while back I also had a grounding issue that introduced digital noise until I tracked it down. Ultimately I spent 50 cents buying a 3 to 2 prong plug adapter and everything cleaned up immaculately.