That's the first heart sound, or S1.

这就是第一心音，或者S1。

That first heart sound, " lub" , is called S1, and the noise comes from the tricuspid and mitral valves snapping shut when the left and right ventricles contract, which happens at about the same time.

第一心音“lub”叫做S1，是在左、右心室同时收缩，三尖瓣和二尖瓣关闭时发出的声音。