Where do sounds of heartbeat come from?

The first sound you hear, the one which says ‘lub’ is caused by the rapid closure of the valve between the auricles and the ventricles (auriculo-ventricular valves). This sound, rather like a door closing, is accompanied by a dull murmur, caused by the contraction of the ventricular muscles.

The second sound is caused by the closing of the arterial valves. As for the third sound, this corresponds to the arrival of the blood being expelled forcefully from the auricles into the ventricles.