I am in the exact same situation with an Eris 4.5 Bt and a PC (Minisforum UM590). I have debugged until I am blue in the face. I'll spare you the details of all the trials and errors with the Mackie board (I thought that might be the problem) but ultimately did the simplest test of all. I connected a 3.5 stereo to 3.5 stereo directly from the UM590 to the Sonus Aux In. I still get that electrical, 60Hx pulsating static. Please note that I have eight other computers in the room and this is the only computer causing the problem so it must be the computer itself, but why only with the Sonus? If I plug a headphone into the PC, all is good. If I plug the PC into my Mackie and then go to other speakers, all good. But the PC and the speakers directly (or through the Mackie) and I get the electrical interference. Going nuts.