I had this problem once ... I tracked it down to different cords (e.g., guitar, power, headphones, etc.) crossing over each other - I guess some were not as well shielded as others. Try separating all your cords so that none touch. It might solve your problem. If not, try switching out your cords one at a time. Clearly the issue is not with the audio box if it sounds great on the Mac but not your other computer.