It is a mechanical noise from the transformer. The transformer hums with the speaker selector "on" or "off." I did not try it to see if there is a difference with the speakers disconnected.

USB Noise Update:

I received and connected the Schiit Wyrd decrapifier. The verdict...

It's a keeper. Prior to connecting, the USB noise was first noticed with the volume control set at 25% on up. Now the volume has to be cranked beyond 65% to first notice the noise. Listening through headphones I will rarely have the volume approach 50% and most often around 25% with the headphone level control set at 50%. So for all practical purposes, my problem is solved. Not being proficient at estimating db, but I'll gander a guess that I gained about 10db to 15db of USB distortion-free volume (S/N ratio.)

My suggestion to anyone with annoying USB noise, after you try a few different computer USB ports and a couple of different cables and the noise is still there, try the decrapifier though Amazon. You can return it if it doesn't work for you. But there's a really good chance it will.

Have you had a chance to listen to any high resolution files? I would be interested in hearing your thoughts to compare with what others have had to say.

Ground loops are tough to kill and switching power supplies in desktop computers can be the worst offenders of the bunch.

What characteristics should I listen for? My initial impression if pressed for an opinion is that the sound is a bit cleaner and more detailed. But I am not a trained supercritical audioholic type of listener (except for my noise aversion) so I might be missing something. Any differences are likly very minor and only discernable in an a/b test which is hard to do switching cables.

I can't imagine that eliminating several decibels of underlying noise wouldn't clean up the whole package a bit.