Do the following, it's my empiric knowedge but always work for me, and the ground loop goes away!
Wrap a wire around the base of the 3.5m.m. plug, and connect it to the point where the ground of the speaker meets the 0V rail
Make sure your computer, printer etc. are connected to the same outlet, and properly grounded. I swear it, I was picking noise through my printer USB connection
Check the attached drawing
ps: Also solved the ground loop when I have connected my DVD player and TV to my computer's LINE IN, if anyone has the same problem