Symptoms of Pneumomediastinum
The main symptom is normally a severe pain in the center of the chest. Other symptoms can include:

air under the skin on the chest, known as subcutaneous emphysema
changes in the voice
coughing
difficulty swallowing, known as dysphagia
labored breathing
neck pain
shortness of breath
vomiting

A doctor, listening to the chest of someone with a pneumomediastinum, may hear a crunching noise that is in time with the heartbeat. This sound is known as Hamman's crunch.