Eat and drink
With every mouthful of food or drink, you swallow a couple of millilitres of air – mostly nitrogen and oxygen. Carbonated drinks also add another millilitre or so of carbon dioxide.
The gases separate from the food in your stomach and press against the lower oesophageal sphincter, which holds the top of the stomach closed.
Eventually, the pressure forces the sphincter open and the air rushes out. The belching sound is caused by vibrations in the sphincter and oesophagus wall