Why does your pool heater make a cracking noise?

A pool heater may make a cracking noise due to thermal expansion and contraction of its components as they heat up and cool down. Additionally, sediment buildup in the heat exchanger can cause popping sounds as water bubbles through the debris. Another possibility is the expansion of metal parts, which can create noise as they shift during operation. If the noise is excessive or concerning, it may be wise to consult a professional for inspection.