Drumming sound when knocking on the left rib cage is mostly considered to be caused by pneumothorax, cavernous pulmonary tuberculosis, emphysema and other reasons. 1. Pneumothorax: Normal pleural cavity is a closed potential cavity without air, if the gas enters into the pleural cavity, resulting in the accumulation of gas, there may be a drumming sound when knocking. 2. Cavity-type tuberculosis: air flows in the cavity after the formation of cavity, when the cavity is large, there may be drumming sound when knocking. 3. Emphysema: If you suffer from emphysema, the air content of your lungs will increase and you may experience a drumming sound when you knock. It is recommended that those who have drumming sound when knocking on the left rib cage should consult the doctor in time to clarify the cause of the disease, and carry out standardized treatment under the guidance of the doctor to avoid delaying the condition.