I flushed the WH tank out yesterday and it did reduce the noise considerably.
Our WH is a Dometic which, as I understand it, is Atwood as Dometic bought out Atwood. That is the only name I can find anywhere on the unit or the paperwork. This water heater does not have an annode to be replaced.
There are a few reasons why this WH needed cleaning as far as I can discern. 1) We have been Full-Time in this rig since the last week of May. 2) The park we've been in since the end of June has some very hard water. Many of the Long Timers here have installed water softeners for that reason. I was amazed at all of the calcium that washed out of my water heater.
I used Gas and Electric together only the one time so the water would heat faster, which it did. At all other times I have only had the WH on Electric heat. I would have expected to hear the thing more had I been heating my water with gas. The electric option has worked great as saved me a lot of money on propane.