The rumbling sound that you hear originates in the stomach and the small intestine. The wall of the digestive tract is lined with smooth muscles. When food enters the system, these muscles contract to propel the food forward and mix it with the gases and juices. These contractions produce vibrations and the rumbling noise.
These muscles also contract when the body senses the absence of food. Hence the rumbling sound occurs not only after you have had a meal but also when you feel hungry. But the sounds are louder when the stomach is empty. It is like shaking a carton of juice that is full versus one which is almost empty – the empty carton makes a louder sloshing sound.
Ancient Greeks had a name for this rumbling – they called it borborygmi.
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