The stomach rumbling sounds are known as borborygmi, and they actually emanate from the small intestine as well as the stomach.
What is borborygmi?
They occur when the gut muscles contract and vigorously move around the contents – food, liquid, and especially gas inside the hollow organ of the gut. The gas may be present for a number of reasons such as swallowing of air during eating and drinking, from carbonated drinks, or production by gut bacteria. Many people experience borborygmi, particularly after a meal, when the gut contains the most food, liquid, and gas, and when the gut muscles are actively mixing and moving the meal through the stomach and intestines. However, these sounds are usually muted by the food and liquid present.
Why does a stomach growl?
During prolonged periods of fasting between meals, a different pattern of movement occurs in the gut known as the migrating myoelectric (or motor) complex (MMC). This consists of strong waves of contraction that start in the stomach and pass right down the gut as far as the large intestine (colon), at intervals of about 90 minutes. The purpose of the MMC is to sweep any residual food, gut secretions, bacteria, and other debris down the gut towards the colon in a kind of housekeeping exercise in preparation for the next meal. The sounds made by the movement of gas through the hollow gut resonate much more loudly in the absence of much food or liquid.
So although the stomach can ‘rumble’ at any time, the sounds are much more noticeable in the absence of food, when we are likely to be hungry. The MMC pattern of activity usually ceases on the ingestion of further food.