I also frequently hear a "clunk" from the area of the Girard when I open a fixture. And if the tankless heater on/off switch is on, it will sometimes even start the vent fan when it does this, as if the burner is preparing to fire. The fan runs for a few seconds and then quits. This is even though I am only using cold water somewhere in the coach. I thought it might be the W/H flow switch, but I am surprised it would have such a specific sound. Maybe pressure variations?
I have my Seneca at the local Jayco dealer right now looking at the water heater, and you jogged my memory to include this information to them. I don't know if it might have a bearing on my issue where the W/H cannot seem to deliver a constant water temperature, regardless of what we try. Followed all the tips in the manual, and other suggestions I found in this forum but it never settles down. I waste a lot of water, which is unacceptable since I camp in state parks frequently which (mostly) do not have water and sewer hookups.
I wish I could have opted for a traditional tank-type unit, I never had any issues with those in my two previous MHs.