Ground loop hum

Congratulations! Glad to hear you found it. 

This is one of the great reasons to post your system under virtual systems. I bet this would have been recommended very quickly if someone had seen photos of your system. Words unfortunately only identify the things you focus on... while a photo will show things you may not have thought of. 

Do you have parts of your system plugged into a different duplex receptacle from the rest? That can create a ground loop. Plug everything into one (you'll have to use a power conditioner or power strips). If it goes away... that the problem. I had one long ago. I put in two direct lines and obviously that fixed it.