pogo could be right, the sender might be stuck down low (or stuffed altogether) causing the buzzer to go off. I had to get a new fuel sender (very costly for my boat) as it constantly showd a 3/4 full fuel tank.
Ben, when i had the buzzer going off i rang my local dealer who services my boat and they told me the problem over the phone(as i said, turned out to be low on gear oil), try doing the same as it will save you taking it to them to get charged just so they can have a look.