A Matter of Quiet.
Eventually, Hodmandod Senior came up with an answer: traffic. A distant rumble of internal combustion engines. With the windows open it sounded something like the roar of the sea, but when closed it seemed that the double glazing of the new windows changed the frequency of this sound into something else. We packed the window with layers of old curtains and hardboard which muffled it, but it was still there. Still there until today, when our window fitters swapped the double-glazed units of standard glass for acoustic ones. The glass is thicker, there's a film of plastic and a wider space between the two panes.
When he'd finished, we went up to listen. The option we had chosen was the cheapest one on offer, although it was still expensive given that we'd only just had the window replaced. Also, there was no guarantee that it would work - but it has. I go into the room and listen. All is quiet. I keep waiting for the sound but there seems to be nothing there at all. Maybe, just maybe, it is a little too quiet.