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 be attributed to greater air flow than the duct work is able to manage. Have a qualified company analyze your home's ventilation system for proper design and make necessary recommendations. Improper duct work layout can be attributed to a variety of ventilation and air quality problems, including: 

1. excessive noise
2. higher energy usage
3. lowered life expectancy 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 undersized you may have other concerns, such as leaky ducts and poor layout that reduces airflow. In humid states this can encourage mold growth within your ductwork.