Wheezing is the whistling sound of air moving through inflamed airways in the lungs. Inflammation causes the walls of the airways (bronchioles) in the lungs to swell and push inward, making the opening that air flows through much smaller than normal. The smaller space causes the air that flows through to whistle, just like the sound we make when we purse our lips and exhale.
Wheezing is usually more than a single whistle. Many airways will be inflamed at once, causing a harmony of whistling sounds much like an entire crowd whistling all at once. Most of the time, wheezing is only heard with a stethoscope placed on the chest. In severe cases, wheezing can be heard without a stethoscope. It's also common for the victim of wheezing to hear it.