Couple of possibilities. My first thought would be the temperature/pressure valve. With the WH off and cold, "exercise " the valve a few times by using the lever to open and then close. Then turn the heater on and let it come up to temp.
If the sound still returns, then open a hat water faucet and record the water temperature. If the temperature is excessive then the thermostat may be defective.
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Marshall
2012 Laredo 303 TG
2010 F250 LT Super Cab, long bed, 4X4, 6.4 Turbo Diesel
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