I had exactly the same problem. I bought a ground isolator , put it between my audio interface and speakers and the problem was resolved. Before installing the isolator I had to keep the volume on my monitors very, very low for them to be of any use at all. Now I can turn the volume up all the way without problems.

I have seen a poor quality USB cable induce ground related "digital hash" type noise.
I had a Motif ES7 connected via USB... and was hearing this odd noise.
Finally traced it down to the Motif ES7.  Disconnected the USB cable... and the noise was gone.
Replaced the USB cable... reconnected the ES7... and no more noise.

Hmmm... a crappy USB cable????? Never thought of that.  Thanks for the info.
 
Jim