Strange swishing noise coming from computer speakers?

If your speakers are powered by ac, then it is most likely static build-up. Turn off your computer and all peripherals, unplug all components, including the pc. Disconnect the speakers from the pc. Leave everything unplugged for about 30 seconds. This discharges the static build-up. Reconnect your speakers to the pc, and then plug everything back into the power source. Turn your computer back on. Hope this works. This happens to me occionally. Good luck.