A water heater produces a loud popping noise because of calcium and hard water sediment settling at the bottom of the tank. As minerals accumulate into a dense layer over the burner or lower heating element, water trapped beneath this sediment blanket overheats and boils into steam bubbles. When these steam pockets violently force their way up through the mineral crust, they create the characteristic popping or cracking sound. Verification: Flush out accumulated mineral sediment by draining the tank completely and refilling it.
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