moltopiano: .. rhythmic crackle popping sounds ..
This is how I described the sound caused by an inferior USB cable.
When I used the shorter one from the factory it went away .. so I bought a high quality longer one which worked great.
It was hard to track down because it wasn't 'always' there. So connecting different things to different inputs was deceiving.
I did the same thing in the beta testing area; I posted a sample and no one else heard the noise, which helped me track it down. I thought it was the new beta version causing it, which was kind of embarrassing ..
Apparently the focusrite units can be a bit finicky about the cable.
Just my experience which is eerily similar to the OP's.