When a valve closes, sound comes out. Since there are two valves with the heart on either side, respectively, the sound of dub-a-dub comes out.
When a stethoscope is hit against the hit of a breast, a place is changed and a heartbeat can be heard. There is a place where the sound of two valves of the right heart can be heard well, and there is also a place where the sound of the valve of the left heart can be heard well. Anyway, the sound of dub-a dub is heard.