Check your guttering. I had water overflowing from the gutter and getting forced up in to the gap between roof tiles. It was coming in between the upstairs ceiling and loft floor, reaching the chimney breast before dripping down in to my bedroom.

I had the gutters empties (I paid to have hedgehogs put in them so who knows what happened to those). The guy basically pulled a big chunk of turf about 2 foot long and the problem was solved.

If one of those two walls is an external wall with guttering (or old brickwork), it could be the problem. Sounds like it is if it's a room with a roof above it.