The Muscular Tube
Your digestive system is basically one long, muscular tube. To move food from one end to the other, the muscles in your stomach and intestines constantly squeeze and relax.
This wave-like motion is called peristalsis. It’s like squeezing a tube of toothpaste to get the last bit out.
A Mix of Gas and Liquid
Your stomach isn't just full of food; it's also full of liquids (like acids and water) and gases (like air you swallowed).
When the muscles squeeze, they push this mixture of gas and liquid around. The 'growling' sound is actually the sound of air bubbles popping and liquid splashing against the walls of your intestines.
The Hunger Signal
Why is it louder when you're hungry? When your stomach is empty, there is no food to 'muffle' the sound. It’s like the difference between shouting into a pillow versus shouting into an empty room.
About two hours after your stomach empties, it begins a 'housekeeping' cycle to clear out any leftover bits of food, which creates those famous loud rumbles.
The Housekeeping Wave
This housekeeping process is called the Migrating Motor Complex (MMC). It is a very strong wave of muscle contractions that acts like a biological broom.