Often times, it is 60 Hz noise being radiated from electrical lines interfering with your speakers .
Usually these type of problems are caused by grounding issues and are easily fixed.
1.) Your probably already know this, but your speakers must be powered on the same electrical outlet / surge supressor / outlet strip as your PC. This will help you avoid grounding problems.
2.) Is the audio line from your speakers to your computer ran next to any florescent lights, 120V power cords, large motors, cell phones, ect? Try moving the physical location of your audio line and see if it is a interference problem.
3.) Check for grounding problems:
a.) Take a wire and touch one end to the chassis ground of your PSU, and the other end to the chassis ground of your sound card (or motherboard if you have on board sound), does the noise go away?
b.) Take a wire and touch from one end of the chassis ground of your PC, and the other end to the chassis ground of your speakers. Does the noise go away?
If it's a grounding problem let me know, and we can figure out the best way to fix it.
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