Truma Furnace: Basketball / Thumping Sound on Startup

[DunnYet]

Yep - me again. Reset 2.0 / Hull 1365 2023 LE II.

We are week into a two week loop, and I seem to be finding all the weird noises in the trailer. This one is the furnace. Rebecca describes it as "someone bouncing a basketball."

When the heater first comes on - regardless of just being turned on or cycling based on the temp, there is a metalic flapping sound that repeats for a minute to two minutes under the curb side bed. It sounds like something flapping up and down to me, Rebecca is sticking with her basketball metaphor.

It does not happen everytime, but I can't establish any type of pattern so far. Sometimes it happens as the furnace starts, just now it make two thumps as the furnace shut down.

Any thoughts or suggestions on what to check welcome. It's a maintenance / laundry day tomorrow, so good time to look at it.