ZQA297/30's post reminds me of my first question on PPRuNe, to which I received plenty of interesting suggestions, but never a definitive answer.
There are other things that can cause "beats". Air cycle machines, hydraulic pumps, and inverters all have characteristic whines, hums, and whistles. Even the outflow valves can produce rumbles and hums.
Some of the items are way off the basic engine frequency, but their harmonics can cause odd effects. There are so many permutations possible, many of them transient, that you can be quite entertained trying to pin them down.
Good luck
I live in SW London, pretty much under the flight path to acquire the GS for 27L and have been watching and listening to aeroplanes since I was a kid. Concorde was delightful treat every evening. Mostly, of course, it is a succession of B7x7 and A3xx. What has always puzzled me - my 767 driver nephew and my 73NG driver friend as well - is a brief noise,
occasionally heard, from
some aircraft when overhead my house.
It
seems to occur when throttles are pulled back, or
perhaps on some change in configuration and takes the form of a single, quite loud
groan, very like the noise a wooden door might make scraping across the floor. It only happens once, I cannot narrow it down to a single type or even a single manufacturer and it doesn't always happen. In fact I haven't heard it recently, but then I haven't spent a lot of time outside, so that might be why.
Being a Scout Leader, I have also heard this noise when camping at Broadstone Warren on the A22 south of Forest Row in Sussex - approach to LGW at a guess. Any ideas anyone?
Roger.