The answer to 'air duct smells bad' is that there are several common causes, including mold, dust buildup, pests, or decomposing organic matter. Symptoms may include a musty or unpleasant odor coming from the vents or air registers. Solutions can range from simple DIY cleaning to professional duct cleaning, depending on the severity of the issue.
For minor odors, you can try vacuuming or wiping down the visible portions of the air ducts. However, if the smell persists or is accompanied by visible mold or debris, it's best to call in a professional HVAC cleaning service to thoroughly inspect and clean the entire duct system.