Trying to find that noise.

Taking the boards down hasn't stopped it. I'm still hearing that scraping noise, and it's beginning to drive me mad. I keep interrupting whatever I'm doing to go downstairs and listen, then coming back up without having found anything.

I wondered whether an animal had got into a space between the walls. I suppose it's possible, though I'd expect some scratching as well. This sounds heavier. I'm almost certain it's coming from the basement, but it's difficult to place a noise when you're hearing it through the floor.

Today I carried one of the kitchen chairs downstairs and sat in the middle of the basement with a book. I wanted to be there when it happened. I had a coat on and brought a mug of coffee, which was cold long before I finished it.

I stayed down there for four hours. Read quite a bit, kept stopping to listen, got up a few times because the chair was uncomfortable. Not one scrape. Eventually I decided I'd wasted enough of the afternoon and started upstairs.

It happened when I was partway up.

I stopped and turned around. This time I could place it. It had come from the alcove, or somewhere immediately behind it. I went back down and stood in front of the opening. There was nothing to see, and it had gone quiet again.

I started checking the back more carefully. When I put my hand near one of the joints between the bricks, I could feel a little current of cold air. There was a narrow gap in the mortar. I'd missed it when I took the boards off.

I went upstairs for a screwdriver and scraped at the joint. Some of the mortar came out quite easily. I worked a little more loose until there was a hole I could get the end of the screwdriver through. Air was coming out steadily by then, much more than I'd expected from such a small opening. I could feel it against my face when I leaned in.

I tried looking through, but it was too dark. Went back upstairs for the flashlight, then spent a while crouching in front of the wall, trying to get the light and my eye lined up with the hole.

I couldn't see anything beyond it. The light caught the broken edges of the mortar, but I couldn't find an angle that showed me what was behind the bricks. I tried from both sides and got the same result.

There must be a space back there for that much air to come through. I don't know how big it is or where it leads. I've left the chair downstairs and put the screwdriver and flashlight on it. I'll need to open the joint a bit more to get a proper look.