Why do heartbeats make noise?

It is impossible to put your ear to your chest to listen to your own heart beating. And not everybody has a stethoscope to listen to his neighbour's. However, films and documentaries have let you hear the very characteristic rhythmic beat, which modern composers have sometimes imitated in their music. It is the heart valves that produce the familiar double heart-beat sound, known to doctors as 'lub' and 'dup'. The first sound which lasts for approximately one-tenth of a second, is dull and muffled; the second, which follows fairly closely, is shorter and sharper. These two sounds are followed by a relative silence (approximately half a second for a person at rest). Sometimes a third sound is heard, one-tenth of a second after the second sound.