Not necessary. Once the WH is off, the water will not expand.Could I use up the hot water and then do the test instead of letting the water cool down?
The thought about keeping the WH off for a longer period had to do with what if there is no howl. Is that lack of howl just because it is intermittent, or will there never be a howl with the WH is not expanding water.
It's my guess that the howl will still happen with the WH off. My guesses are often wrong. My thinking is that the howl occurs while a little water is being used, so no thermal expansion overpressure should exist.