It sounds akin to it arrives from the speakers/headphones. If it is so, it might be that you have a cell phone near. It occurs to me, and I have these rotten speakers, they actually bug me... If you have quality speakers, this should not happen. My speakers caches cell phone "traffic" (for instance sms, mms, internet, calls, etc) yet if it is some meters away and trough brick walls.

BTW, the "detector" is at my office, at home I have an USB Live 24! w/ 5.1 Logitech speakers and this does not occur, unless it’s really near the speaker amplifier (on the subwoofer box). Oh and also counts on the frequency of the cell phone hauler band. I think my cell phone is 850, I think higher frequencies make less interference.