Crackles are the clicking, rattling, or crackling noises that may be made by one or both lungs of a human with a respiratory disease during inhalation. They are usually heard only with a stethoscope ("on auscultation"). Pulmonary crackles are abnormal breath sounds that were formerly referred to as rales. Crackles are caused by the "popping open" of small airways and alveoli collapsed by fluid, exudate, or lack of aeration during expiration.