If you're feeling pain or discomfort in your GI tract, it could be something as unworrisome as simple indigestion, or maybe a stress ulcer. Or it could be cancer. For example, your doctor may say you're suffering from non-ulcer dyspepsia. Some symptoms include upper abdominal pain, nausea, vomiting, bloating, and indigestion. Indigestion is another fuzzy word.
Some people like to call it sour stomach, or acid indigestion, or upset stomach, or acid stomach.
Then there's heartburn, which is another matter. Normally, the pressure in your stomach is lower than the pressure in your esophagus, which helps prevent food from reentering the esophagus. But once in a while the delicate pressure system can break down. But whether you make a noise or not, you feel the burning. The lining of your stomach is fairly accustomed to an acid environment, but your esophagus definitely isn't, so even a little acid in there will sometimes be enough to get your attention.