A heartbeat is the sounds that the heart makes when it pumps blood. A change in heart sounds might mean illness. There are normally four heart sounds. Two of them, S1 and S2, can be heard with a stethoscope. Two other sounds, S3 and S4, can not be heard, but they can be seen using a machine called a phonocardiograph. There are many other heart sounds which are abnormal and can be heard or seen. The heartbeat can be normal until 100 beats per second.