The "thump" of the heart is the sound of the heart valves closing At first (first "thump") the mitral valve, located between the left atrium and left ventricle, and the tricuspid valve, located between the right atrium and right ventricle, close; Then (the second "knock"), the aortic valve, which closes the opening in the left ventricle, and the pulmonary trunk valve, which closes the opening in the right ventricle.