Aeroplane noises.
Here's a real geeky question, but a genuine one I have pondered for many years. I live under one of the LHR approach routes from, I think, the Biggin and Epsom stacks. They pass over my house on their way to acquire 27L - again I speculate - I'm sure you know roughly where I mean.
Not only do I often stop and watch as they pass over me at about 5000', but I listen too and apart from the usual low power jet scream, and the occasional sound of engines spooling up, some of the aircraft make the most peculiar noise, just once. I have never been able to correlate the noise with any particular type - they all seem to do it, sometimes.
It seems to happen when the aircraft is slowing down and takes the form of a loud groan, like a wooden door that is tight in its frame. It only happens once and it doesn't always happen, but its always from aeroplanes maneouvering to acquire the ILS. When we camp at Broadstone Warren Scout camp, you can hear aeroplanes on their way in to LGW.
I've asked my nephew - a B757/767 F/O - and he doesn't know. Any ideas? (I told you it was geeky)
Roger.