There are several reasons why your ductwork is making a popping sound. The most common reason is when the metal sheet in the ducts expands to accommodate the change in air pressure. While this occurrence is not that detrimental to your system, sometimes, popping sounds from your ductwork also indicate that the duct system is undersized. If this is the case, we recommend that you contact Cassel Home Comfort Heating & Cooling so we can analyze your ductwork system for proper design and recommend adjustments.