I just replaced my indirect -fired hot water tank (Ametrol-classic series, 41 gallon) with identical model due to leaking issues. I never had a problem with noise or humming until I got the new one installed.Contractor used the same pipes and connectors.
it makes the noise daily (5-7 times) during the day and night (it wakes us up all the time). The vibrating pipe is the cold water pipe that is supplying the hot water tank with cold water.
The contractor had no idea and he is not helpful and said that the manufacturing company never heard of such a problem!
I need help with this it is making our lives very miserable:(