It is not a constant sound because several alterations are perceived during its reproduction, there are moments when it is soft, strong, slow, etc. It was activated the moment a person chopped potatoes.
Cleaning up some dialog on a film here is the heart beat picked up by the 2 actresses cuddling, only a second or so - but you probably couldnt have planted the mic better on purpose!!.
This only happend to me once in my life and it hurt(i went to the hospital after this attack). I used a device that can give me almost the clearest possible sound of my heart beat. Also i gave the very tiny parts where my heart was beating at 400-600 an extreme amplification because those parts are very quite.