A knocking noise coming from your water heater is typically caused by thermal expansion or heavy mineral sediment boiling inside the tank. As hot water is generated during heating cycles, metal components, internal piping, and steel tank walls expand and rub against structural framing or pipe hangers, creating sharp knocking sounds. Alternatively, if hard water sediment builds up over the heating elements, trapped boiling water pockets create hydraulic knocking vibrations. Verification: Inspect pipe connections for thermal expansion movement and flush the tank to remove sediment scale.
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