Does this get worse after sundown? If so, you`re picking up Medium Wave transmissions. They bounce around the globe under certain conditions. Or it could be that there is a radio transmitting mast near to you and its close proximity means you`re in the "fallout" area where the signal is overpowering. Buy some ferrite rings and try them on in turn your mains, signal and speaker cables. Find by elimination which cables are acting like antennas. Are you using a set of el-cheapo PC speakers?? the screening on this is worse than abysmal. Hope this helps.
I had this problem a couple of years ago with a PC I built...it only ever happened late at night at around 2-3am...I would hear a ruffle then when I turn up the volume fully I can hear a radio station...I thought it was really funny cos that had never happened before. so I wouldnt really worry about it unless its really loud and doing your head in!