There's a persistent whiny noise in the air near us. It comes and goes but it's more coming than going at the moment.
It's only mildly annoying in our flat, the sort of noise that bugs you when you can't sleep but doesn't wake you up.
It's hard to record, but here's a stab done with my phone. It's very quiet on the recording. Louder in real life. It's a fairly pure sounding A♭. Pretty sinewavey.
It's been around for several months now. We assumed, when it started, that it was coming from the roof of our building. But, the Council Noise Team having been called, it turns out that it can't be pinpointed. Everyone thinks it's coming from the roof of their building. And there are so many buildings and echoes round here that it can't be traced. Which is just weird. You'd think that it was close enough to someone that they'd be complaining extra hard and it would be tracked down, but apparently not.
(All the info above, by the way, I'm basing on what I've heard from Anne about what the residents association mailing list rumours say, so it might not actually be true.)
Of course, it could be submarines or hill-spirits but I suspect it's not The Hum, since everyone can hear it and it's not that low.