When I switch on the sound on my pc,there is a faint but discernable regular noise like a pulse or heartbeat in the background, my son says it sounds like a record turntable when the stylus is running in the leadout track. It is always there even when there are no other connections. The sound card is S/blaster audigy,and using Win XP.Has anyone experienced this? is it the card or my sound system? which other than this noise performs great! Any help please?