Has your stomach ever growled in a quiet classroom? The sound comes from gas and liquid moving through your stomach and intestines. When your stomach is empty, your gut does a kind of cleaning.

Every one and a half to two hours, a strong wave pushes old food and bacteria forward. The growling is loudest during these waves. A hormone called ghrelin, found by a Japanese team, tells your brain you are hungry.

Stomach growling is not a sign that something is wrong. The stomach and intestines are like muscular hoses that squeeze in waves to move their contents forward. When gas and liquid are mixed inside, this movement makes noise. After a meal, food softens the sound, but when the stomach is empty, only gas and liquid move, so the noise is louder.

When we have not eaten for a while, the gut also performs regular cleaning waves, which push leftover food and bacteria along every one and a half to two hours. A 1999 Japanese study recorded stomach sounds and found that they were loudest during the strongest of these waves. Eating stops these waves.

The rumbling of an empty stomach, embarrassing as it may be, is evidence of a busy gut. The stomach and intestines move their contents by peristalsis, waves of muscular contraction, and when gas and liquid are pushed along together, the result is audible. After a meal, food dampens these sounds, but when the gut is empty only gas and fluid move, making noise more noticeable. In 1969, Szurszewski reported that in fasting dogs, waves of electrical activity traveled regularly down the small intestine. This pattern, later named the migrating motor complex, acts like a housekeeper, sweeping away leftover food, bacteria and dead cells roughly every one and a half to two hours in humans.

In 1999, Tomomasa and colleagues placed microphones on the abdomens of fasting volunteers and found that bowel sounds were loudest during the most intense phase of this cycle. Hormones also play a part: ghrelin, discovered by a Japanese team in 1999, signals hunger and may strengthen these movements. Note, however, that the sound study involved few participants, and some suggested links, such as weak cleaning waves leading to bacterial overgrowth, are not fully established.