It is sediment, and it is information. Hard water minerals settle on the tank floor; the burner or element then cooks water trapped under that blanket, and the popping is small steam events under the sediment. The tank is not about to explode, but it is working harder, heating slower, and aging faster, and a tank that has reached the rumbling stage in this water is usually in its last act. A flush can help a younger tank. On a tank past ten winters, price the replacement as planned work while it still heats, because the failure that follows the rumble, in this climate, prefers a cold morning.
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