Thanks. EH has told me even if they witnessed it themselves, unless we know where it comes from they could not question the neighbours! I have a really hyper friend who could come and stay, no way would he put up with it, he'd be banging on doors at 3am, lol. He has no boundaries. The only thing is I'm too exhausted to have him over, lol, he's hard work, but he'd be perfect for this. Maybe soon. The economy 7 thing seems likely... The overflow pipes are no longer leaking out water (one was gushing, the flat beneath me, but has been solved now after I complained) - as you say unlikely to cause a prolonged hum though... I don't think the flats have a roof top reservoir, I think the water comes straight up from the ground (the flats are only 2 floors high)... Re in-flat booster pumps, as you say makes no sense that they would be running at night... So to me, the only thing that rings true is something to do with water heating using cheap rate electric. In a week or two the downstairs neighbours will have moved out, that will mean only one adjoining flat is occupied (my side neighbour). I'll see what happens then.