Silence
You probably already know about the experience of John Cage in the anechoic chamber at Harvard University, a room especially designed to completely absorb every single sound. In that occasion, Cage entered the room persuaded to be able to hear silence, but he discovered the impossibility of it instead. In fact, even in that extreme condition, two sounds were present: the low frequency sound of the blood in circulation and the high frequency sound of the nervous system. That experience led to the composition of 4'33''.
It is fascinating to think of our body as the primordial, true instrument.