If you’ve ever listened to a heartbeat through a stethoscope, you’ve heard the classic rhythm: Lub-Dub, Lub-Dub, Lub-Dub . While both sounds are fascinating, this guide focuses on the first one: the “Lub.” In a Nutshell: The Short Answer The “Lub” sound is caused by the turbulent closing of the atrioventricular (AV) valves —specifically the tricuspid valve (on the right side of the heart) and the mitral valve (on the left side of the heart)—at the beginning of ventricular contraction (systole).