Noise coming from gas models can often be caused by the sediment build up in the bottom of the tank. The heating of water causes calcium carbonate to precipitate out and settle to the bottom of the tank. Steam bubbles form under the sediment when the burners come on causing popping and other noises as the bubbles escape from under the sediment. Sediment buildup depends upon what kind of water you have. In some areas there is virtually no sediment and in other areas sediment buildup can be significant. Regular flushing of your hot water heater helps prevent sediment build up.