[ Update: I went to the back side of the walls, ie exterior. And updated a few more photos. Possibly the cause ]
When I put my ear against the wall, I think I hear something.
Is this because of termites ?
Adding three more photos for more context. First show inside of house, second size, its next to the fireplace. And one to show the back of it, where we have brick walls.
==== The wood looks damaged, possibly from water ===