This sounds like some kind of interference jumping into a speaker line.
Does it only happen with your headphones, or do regular speakers do it too? It could be a frayed wire like zoopa said.
Try swapping speakers\headphones, try swapping the audio cables. And if it is being played through a real stereo, try plugging the computer stuff into a different wall plug than the stereo stuff. Turn off all TVs, vacuums, etc in the house and then give it a listen.
I've had this problem with every computer I have had- My house has really old wiring.
Of course, if it is a computer issue and not an inteference issue my advice won't do a damn bit of good : )
-the monster