An ultrasonic scaler uses vibrations to dislodge larger pieces of tartar/calculus deposits loose. At the same time as vibrating it sprays a mist of water. This washes away the debris to stop it re-sticking to your teeth and keeps the area from heating up and being uncomfortable.
This dental instrument often makes a humming or high pitched whistling sound. That sounds very loud to you as the sound gets amplified by being inside your mouth. In the same way an electric toothbrush sounds really loud to you when it’s inside your mouth when brushing your teeth.