A knocking sound in a water heater is usually caused by thermal expansion or heavy mineral sediment boiling inside the tank. As hot water is generated, metal components, piping, and tank walls expand and contract against framing or structural connections, creating sharp knocking noises. Alternatively, if sediment builds up over the heating elements, boiling water pockets create hydraulic knocking vibrations. Verification: Check pipe connections for thermal expansion movement and flush the tank to remove sediment scale.
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