At 1am last night there was a nosie that woke me up. It was a single loud click. It sounded (to me, in my paranoid state) like the sound you would get if you pulled down a door handle then let go so it sprang back into place. I was immediately awake and checking at the windows. There didn't seem to be anybody around, front or back.
Interesting that it should happen at exactly 1am. Perhaps something in the house switches over at that time. I thought perhaps the video I left running had reached the end and ejected itself. No. I'm not aware of anything the heating is supposed to do at 1am. I wondered whether the bag of rubbish hung inside the bathroom cabinet had fallen. No. So I still don't know what it was. Five minutes of investigation and I was ready to go back to bed. I'm going to wonder what that was for a long time, though.