Borborygmus is the scientific or medical term for a stomach rumble, a.k.a. bowel sound. It is the growling or gurgling noise produced by your small intestine when the contents of the gastro-intestinal tract are pushed through by muscle contractions. 

While a doctor may listen to the borborygmus noises with the help of a stethoscope, the rumbling can also be loud enough to be heard by you with bare ears.