Hmmm... have you tried unpluging everything (including the battery) and just plugging in the power cord. Try hooking it up to a different outlet (one directly to the wall, no power strips/protectors) see if it still does it. If it doesn't add the battery to the mix then just keep adding until you hear it again.
Edit: also keep any other electronics away from the laptop. Cellphones, wiring and other things could be causing undesired interference.
If nothing helped, i'd take it back to where you got it from.