Per NLM in 2014, "most bowel sounds are harmless and normal. They simply mean that the gastrointestinal tract is working. A health care provider can check abdominal sounds by listening to the abdomen with a stethoscope (auscultation)." Stomach rumblings can be caused by gas moving through the intestines which is referred to as borborygmi.
Loud borborygmi or rumblings may result from contractions of the intestines caused by diseases like Crohn’s disease or bowel obstruction.
Per Mark A. W. Andrews in Scientific American 2002, "The physiological origin of this rumbling involves muscular activity in the stomach and small intestines. When the walls are activated and squeeze the tract's contents to mix and propel food, gas and fluids through the stomach and small intestines, it generates a rumbling noise. This squeezing of the muscular walls is termed peristalsis and involves a ring of contraction moving aborally (away from the oral cavity) towards the anus a few inches at a time.
"The generation of these waves of peristalsis results from a rhythmic fluctuation of electrical potential in the smooth muscle cells, which, all other conditions being appropriate, will cause the muscle to contract. This fluctuation is called the basic electrical rhythm (BER) and is a result of inherent activity of the enteric nervous system, which is found in the walls of the gut. The BER causes the muscle cells of the stomach and small intestines to activate at a regular rhythm (three and 12 times per minute, respectively), in a manner similar to, but slower than, the rhythmicity of cardiac muscle in the heart. receptors in the walls of the stomach sense the absence of food, causing a reflex generation of waves of electrical activity (migrating myoelectric complexes, or MMCs) in the enteric nervous system. These MMCs travel along the stomach and small intestines and lead to hunger contractions. However, there are some cases in which abnormal sounds can indicate a problem."
There are three kinds of bowel sounds, hypoactive, hyperactive and absent.
Reduced (hypoactive) bowel sounds mean a reduction in the loudness, tone, or regularity of the sounds and may be a sign of slowness of intestinal activity. "Hypoactive bowel sounds are normal during sleep. They also occur normally for a short time after the use of certain medicines and after abdominal surgery. Ileus is a condition in which there is a lack of intestinal activity which may cause gas, fluids, and the contents of the intestines to build up and break open (rupture) the bowel wall."
"Increased (hyperactive) bowel sounds can sometimes be heard even without a stethoscope. Hyperactive bowel sounds mean there is an increase in intestinal activity. This may happen with diarrhea or after eating. No bowel sounds after a period of hyperactive bowel sounds can mean there is a rupture of the intestines, or strangulation of the bowel and death (necrosis) of the bowel tissue. Very high-pitched bowel sounds may be a sign of early bowel obstruction."
The doctor may be unable to hear any bowel sounds when listening to the abdomen which may relate to absent bowel sounds often indicating constipation or a more serious problem of ileus.