The languid, soft "lub dub" that periodically purrs in your ear when you lay your head on the chest of a loved one.
This sound is generated by the leaves of heart valves snapping shut after each has ejected blood from a chamber of your heart.
There are 4 vavles in a human heart... so why are there only two sounds (ie. lub & dub)?
Before we go into this, the 4 valves are called:
Now, of these 4 valves, the tricuspid and mitral valves lie between their respective atrium and ventricle (tricuspid on the right side, mitral on the left). When your heart begins its contractile cycle, the first compartments to contract are the atria. When the blood has been squeezed out of each atrium and into the ventricle below, these valves snap shut to prevent flow back of blood back into the atria. Under normal conditions both the mitral and tricuspid valves snap shut at the same time, hence the one unified sound:
Now, for the final phase of the cycle, the ventricles then contract, squeezing blood into the lungs (from the right side) and into the rest of the body via the aorta (from the left side). The gatekeepers at the exit door of the ventricles act in the same way as the other two - they prevent blood from flowing back into the ventricles after ventricular contraction (aka systole). Hence, after blood has rushed out, these valves snap shut.
So next time you're snuggling your partner and hearing these familiar sounds, appreciate it's not just poetry in motion, but also some damned awesome engineering at play.