When you fast, there is a neurological reflex that causes your body to increase peristalsis (waves of contraction of your intestines) in your gastointestinal tract in an attempt to move any remaining substances through the system. This is called the migrating motility complex. (http://en.wikipedia.org/wiki/Migrating_motor_complex). These additional peristaltic waves contribute to the "growling" you hear when you are hungry. This is the reason that when children swallow inedible objects (like a penny, for example) instead of removing the object, doctors will have the child fast. This activates the migrating motility complex which pushes the object through their intestinal tract (and out their backside)
A lot of people have been asking about where the "sound" itself comes from. The sound comes from the fact that the intestines are muscular tubes. When the muscles contract and then relax, it creates a vacuum. To compare it to something you might be familiar with- when you push on a whoopie cushion it makes noise, but when you release it and let it re-inflate, it makes a different sound from the air moving back into the balloon. Both of those types of noises are going on inside your intestines.
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