Typican ground loop humming problem occurs when you connect your computer sound card to a stereo receiver which is connected to wall antenna plug. The computer is connected to grounded power outlet which causes that the sound card audio ground is connected to mains ground. The receivers gets connected to antenna ground because it is wired to the antenna plug in the wall. The mains ground and antenna are tied together (ar at least should have) somewhere else in the house. Unfortunately there is all kinds of electricla interference which can be coupled to those grounding wires and cause small potential differences between the ground on the antenna outlet and your mains outlet. The small potential difference causes small current flowing through the equipments and cables which causes hummign to them.