What's causing my duct work to pop when the furnace starts or stops?

A popping sound is a typically sign of an under-sized duct design. It may have to do with greater air flow than the duct work can handle. Have a qualified company analyze your home's ventilation system for proper design and recommend adjustments. Improper duct work layout can be attributed to a variety of ventilation and air quality problems, including: 

1. loud banging noises
2. higher energy consumption
3. lowered lifespan of the system
4. uneven airflow throughout your home
5. damaged compressor in summer
6. overheated unit in the cooler months

In addition, if your ductwork is incorrectly sized you may have other problems, such as leaky ducts and poor layout that restricts airflow. In humid states this can encourage mold growth within your ductwork.