Icom-2100, DigiRig Mobile, standard split audio cable (4 conductor 3.5mm phone plug on one end, 3 conductor 3.5mm plug for speaker jack, and standard ethernet type plug for the mic jack). With the Digirig connected to PC via USB cable, and audio cables connected, the Digirig is causing a hum on the Icom. When I unplug the speaker cable, and the volume turned all the way down, there is a loud hum coming out of the Icom speaker. It disappears when none of the audio cables are plugged into the Icom. Also disappears when I unplug the USB cable. I tried a different USB cable, no improvement.
You might want to try a USB isolator. It is possible that the hum is caused by the ground loop.
Grounding the PC and radio to a common ground or ground plane (low impedance/higher gauge than signal wires) might help as well.