Recently, my Rinnai has been making this absurd sound that is most analogous to the roar of the Cloverfield monster terrorising NYC. It only happens when it is first active when trying to get some hot water. I think it's probably a really violent vibration that is causing this. It's definitely being caused by the Rinnai, not the pipes leading to and from, although the pipes may be reverberating from the vibration.
Has anyone else had this issue? Mine is about a 3 year old internal mount model.