The sound you are referring to is called borborygmus.
There are lots of causes for borborygmi, as you can imagine, anytime the bowels and digestive tract are working, they are bound to produce noise.
In actuality, for a "perfectly healthy" person borborygmi are mainly caused by digestion and hunger. Further, the ones caused by digestion can, for the most part, be heard mainly by a stethoscope. Thus, the only audible borborygmi, for a hypothetical, perfectly healthy person, with a perfect diet, who had perfect digestion, would be from hunger. This is of course also assuming that this hypothetical human did not swallow any large amounts of air while talking or eating. The following link details some of the possible underlying causes for borborygmi