Cloverfield monster or Rinnai?

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.

Thanks

So, after a little more sluething, it turns out it's not the cloverfield monster, but a cyborg cow. According to this post, there's a hypothesis that it has something to do with the heat exchanger in the Rinnai and the outside temp (it is cold here).

"If it is particularly cold outside, a sudden "full throttle" hot water demand (e.g. filling the washing machine) will cause a brief but loud vibration sound, not unlike a dying cyborg cow. This seems to be harmless and is most likely caused by thermal expansion of the heat exchanger. That part has a substantial warranty, so I'm not going to worry."