Not long after we installed our new 40 gallon American Standard gas water heater, we noticed a low, fog-horn-like hum, (for lack of a better description), that starts rather quietly, rises in intensity and volume, so that it can be heard throughout the house, and then steadily gets quieter until it becomes silent again. This all lasts maybe 15-20 seconds or so.

Notable is that extremely high water pressure comes into the house and the heater has been fitted with a pressure relief valve.