I would guess that the buzzer is for your Add Coolant light. Is the light coming on? Maybe it's burnt out or something if not. I had this same problem, and fixed it today by bleeding the air out of the coolant system while adding a bit more water. I had the problem happen once before as well. If bleeding the coolant system doesn't work, you can also take the sensor out on the top of the radiator and check it. You can just unscrew it with a 17mm wrench. The Hayne's manual and FSM have procedures for testing the sensor. I believe that if the sensor is grounded, then the coolant buzzer shouldn't go off, and if it isn't grounded it should.