husker
Thanks Abacab.  I think I am going to throw in the towel on this.  The cheapest solution I guess is simply just to turn down those inputs on the Focusrite when I don't need them.  I'm just one of these guys who wants to understand everything, and the whole thing has just really bothered me.
 
This issue has really gotten me sidetracked on what I want to do - learn about Sonar and make some music.

You should check if the noise is also part of the recorded signal (which I would assume it would be). If so, you should still try fix it, otherwise you won't be happy with your recorded quality.

I'd order a simple 2 channel passive DI box with a return policy. Probably only costs you about 50 bucks. You can also test the Nord with it ... if it does not help, send it back...

I had a frustrating similar problem a few years ago. I could make it go away by running a wire metal chassis to metal chassis between my synth and my computer, essentially grounding them together. Another "fix" was when I discovered that several  unused inputs on my Roland  interface had their gain turned up for no reason. Turning down the gain on the unused channels made the hum disappear, even without grounding the synth to the computer. My assumption was that the loop was still there but no longer being amplified. For some reason it was not audible on the channels that have the synth plugged in no matter what the gain on those channels.
 
I am not sure if grounding devices to each other is safe or not. The effect is similar to lifting the ground on one of the devices, though (not an expert by any stretch here) it seems the devices would still be grounded.
 
Brian

https://www.music-group.com/Categories/Behringer/Signal-Processors/DI-Boxes/HD400/p/P0387
That has helped be in 2 cases of USB ground looping:
1) TC GTX -> (audio) -> DP. DP -> (MIDI) -> TC GTX.  As soon as DP was connected by USB to computer, especially when TC GTX was connected to it as well, there was no fun...
2) TD-20 -> (audio) -> Mixer. Mixer -> (USB) -> Computer. As soon as TD-20 was also connected by USB to the (same) computer, there was again no fun...
 
I have different interconnections now, and I have not managed to solve all USB related noise (when TD-20, DP, VS-20 and my mixer all together have USB connection, no matter what I do the result is noisy). But HD400 is cheap, so it may be worse to try.

i'd say it was more related to plug sockets (or adapters) that have that bigger voltage, than any pc parts.
 
you need to eliminate various aspects. e.g. if it's a laptop try without PSU.. if you use a gang socket , cheapo ones here cause issues (first place I looked when finding them for others) , to narrow it down to 1 thing.
 
depends on where you are, as US is obviously different from the UK.
have a read
 
https://support.native-instruments.com/hc/en-us/articles/210293145-How-to-Resolve-Ground-Loop-Issues-Humming-and-Buzzing-Sounds-
 

I appreciate all of your suggestions and help. I think I have finally resolved my issues;  at least enough where I can get back to focusing on what I need to.
 
Basically, I had multiple electrical noises across two devices. 
 
On the Moog, I swapped to from a balanced cable to an unbalanced cable, then used a ground lift adapter on the plug.  The combination of those two seemed to have resolved that issue.
 
On the Nord, I moved the power to the same strip as my interface, then used a USB filter between the device and the computer.
 
Electrical problems suck.