Ok It turned out to be the chassis intruder alarm !
god knows why cos I havnt got it coonected and its never gone off before.
mpooley--I just ran across your article while trying to figure out an annoying high-pitched alarm I started getting. What kind of motherboard do you have? I just put a 6850 into my GA-EX58-UD5 and this is the first time I ever heard the alarm.