I had the idea of using a contact microphone attached to your belly to monitor digestive function.
Hooking it up to the scope, I can see my heartbeat in my wrist quite clearly:
But, interestingly cannot see it in my neck. Attaching it to a headphone amplifier and routing that into headphones reveals some interesting stuff, but the headphone amplifier has an input impedance of apparently 17kR, which is too low to be able to pick up lower frequency content.
Time constant from scope
Here is the response to a well calibrated finger-press square wave input:
So with the 1Mohm scope input, the time constant is ~50ms apparantly.