The popping is steam. Sediment settles at the bottom of the tank, water gets trapped underneath it, and when the burner fires that water flashes to steam and forces its way up through the layer. It sounds alarming and it is mostly a maintenance issue — until it isn’t.
When a flush fixes it
If the tank is under about eight years old and the water still runs clear, draining and flushing usually quiets it down and buys back some efficiency. That sediment layer is insulating the water from the burner, so your recovery time and your gas bill both suffer.
When it means something worse
Rusty water, moisture around the base, or a tank past twelve years changes the calculation. At that point the popping is a symptom of a tank that is near the end, and a flush on a corroded tank can occasionally be the thing that starts a leak.
If you are not sure which one you have, the anode rod tells you a lot. Pulling it takes ten minutes and it is the single best predictor of how much life is left.