Final answer:
During diastole, the 'dub' sound is heard, which is the closing of the semilunar valves, also known as the second heart sound or S2. This sound is part of the normal 'lub-dub' pattern observed with each heartbeat.
Step-by-step explanation:
The sound of the heartbeat heard during diastole is described as a "dub". This sound, or S2, occurs when the semilunar valves close after blood has passed through them. The closing of these valves prevents backflow into the heart during the isovolumetric ventricular relaxation phase, which characterizes the early phase of ventricular diastole. There is no silence during this phase; instead, we hear this distinctive heart sound as pressure in the heart drops below that in the major arteries, and the semilunar valves shut to maintain proper circulation.