A 67-year-old man came to the hospital with a history of shortness of breath on exertion and cough progressing to severe breathlessness at rest over several weeks. Physical exam revealed stridorous sounds in inspiration and expiration heard at the mouth and over the trachea. Lung function tests revealed severe airway obstruction in expiration and probably inspiration. Fiberoptic airway inspection revealed a large mass above and arising from the main carina (first airway bifurcation). This mass obstructed the distal trachea except for a narrow (∼2 mm) circumferential space between the mass and the tracheal wall. Breathing sounds were recorded from in front of the mouth and the anterior neck overlying the trachea both before and after laser ablation of the mass. Before ablation, the tracheal sound exhibited unusual resonances and antiresonances that rose in frequency by nearly 100% during inspiration and fell during expiration. These variable sounds disappeared after laser ablation of the mass. It is postulated that the mass behaved as a dynamic acoustic obstruction and thereby altered the distal boundary condition and resonance behavior of the respiratory tract during breathing. This provides insights into the links between the airway anatomy and measurable acoustic properties of breath sounds.