We report the case of a 14-year-old boy presenting to our unit because of a spontaneous left pneumothorax. Over the 3 preceding weeks, he described a rhythmic precordial click and the sensation of bubbles floating inside his chest. This noise, audible to the patient and his family without a medical device, proved to be the Hamman sign, a rare although typical sound formerly related to pneumomediastinum that can be exceptionally encountered in isolated left pneumothorax. Its pathophysiology remains unclear. Small pockets of air close to the heart may be pushed inside pleura and amplified by the chest wall, hence provoking a recurrent rhythmic sound. A small left pneumothorax must be suspected whenever the Hamman sign is elicited in the absence of pneumomediastinum. Chest tomography must be precociously considered if plain x-rays are not contributive.