Turn the speakers off.
If annoyance continues - then choose different broadband transmitting medium.
If annoyance stops, then chances are that your speakers are running on same power as pc. Put seperate plug on speakers so that they don't run on same power that overutilised PC power supply is running on.
If problem still persists, put PC onto a UPS, so that UPS cleans up PC powersupply feedback.
If problem still persists, then it's nothing to do with the power feedback and you'll have to move the speakers away from the transmitting device.
Though - here in me office, I had same problem, the give'away was that the noise came from both speakers, not from the closest speaker to the RF card. I now run me PC on a ups and the speakers on normal power, they [speakers] were powered by an adaptor attached to the power plug supply thing.