Your water heater is making rumbling noises because of a thick sediment layer at the bottom of the tank, essentially a deeper version of the popping problem.
When the burner fires beneath heavy scale, large volumes of trapped water boil simultaneously and shake the entire tank, producing a sound like distant thunder rolling through the house.
The vibration stresses the glass lining and can accelerate corrosion of the steel underneath.
Begin by shutting off the heat source, connecting a garden hose to the drain valve, and flushing until the water runs completely clear.
If flushing produces little debris, check whether the thermostat is set too high or the tank is oversized for actual demand, since both conditions encourage rapid scaling.
Inspect the anode rod during the same service visit.
Rumbling that continues after a thorough flush suggests the tank is nearing the end of its life.
