If you're too busy to remember to eat, your stomach will remind you it's time to refuel by growling. This is called "borborygmi." Have you ever wondered why it happens?
Your GI tract is powered by involuntary muscle contranctions. After you've eaten, your gastrointestinal muscle contracts and relaxes in order to move food from your stomach into the intestines.
When the GI tract has finished digesting a meal, it continues to process the liquids and gasses remaining in the intestines. This process often causes your stomach to rumble when you're hungry. Stomach growling is caused by intestinal contractions squeezing and popping intestinal gasses. Actually, stomach rumbles are simply flatulence that stays inside the body.
The same contractions that cause your stomach to growl also clean out the GI tract. To see how this cleaning movement works, picture a long hose made of a pliable material. If there were an object, say an egg, at one end of the hose, you could push it from that end to the other by squeezing all along the length of the hose.
This is how the GI tract keeps itself clean. In order to push out bits of food particles left over from a meal, your gastrointestinal tract creates waves of contractions all along the length of the intestines at a rate of two or three per minute.
All of this activity in your GI tract when you're hungry can be very noisy, as anyone who has been in the middle of a pre-lunch meeting when stomachs started rumbling can tell you.
- Stomach Rumble (Wikipedia)