(Oakland, California, USA)
Have a similar A.O. Smith water heater, 50-gallon installed in 2012.
It howls loudly when re-heating water after someone takes a shower, washes the dishes, does the laundry, or all by itself when it senses it's hot water temperature has dropped sufficiently.
It is brand new, installed by a plumber who knew what they were doing, and it functions well except for the noise. It is located one-story below our house in the basement insulated a the distance and structure, and it's still loud enough to wake me up. Plus the noise radiates to the outside through the vent.
We are trying to live with it, but a replacement is being considered in the near future. Also would never purchase an A.O. Smith product again. Wish they would contact me with an offer to replace the unit at some reduced value, that would help.