Over the last few months we have been disturbed by a weird intermittent noise.
It can only be heard in the immediate area of our bedroom wall. This is a party wall with the neighbours (but downstairs is not a party wall because there is a tunnel running between the houses, so there is an external wall downstairs).
The noise only seems to happen between roughly midnight and 8 or 9am, and it's intermittent/fluctuating during those times.
It's a high pitched kind of whining sound - difficult to describe. I could possibly imagine it coming from a pipe (although it's not exactly like any pipe noise I've ever heard). But our heating obviously isn't on at the time, and it seems unlikely the neighbours are putting their heating on all night and only at night. I don't think there would be any water pipes in the party wall (piping to the radiator runs under the floorboards). There is a gas pipe running along the external wall downstairs from the meter, but we have checked and the noise can't be heard outside in the passage, it can only be heard when close to the party wall upstairs.
We've spoken to the neighbour and she couldn't think of anything it could be, and hasn't noticed anything from their side.
We had a theory it could be an ultrasonic device (apparently some people can hear the frequency) but the neighbour said they don't have anything like that.
Any ideas?? It's disturbing us - DH keeps moving to the spare room in the middle of the night when he gets troubled by it.