Second heart sound is caused by
The second heart sound is produced due to closure of aortic and pulmonary valves also called semilunar valves.
AV valves closer i.e. mitral and tricuspid valves closer produces a first heart sound.
Third heart sound which may be heard in young adult males, is due to vibrations of ventricles walls during the rapid filling phase.
Atrial systole may lead to retrograde flow into vena cava
Fourth heart sound happens in stiff ventricles during atrial systole i.e. at the end of ventricular diastole