Quote from Landroger:
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... 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?
Hi, I'm afraid that, as you would expect, I'm not going to provide "a definitive answer" to this one! I've wondered about these noises for about 45 years, typically from One-Elevens flying over Ashdown Forest (as you say) on base-leg to LGW Rwy 27 (sorry, 26). In that case, it was a fairly high-pitched scream as the flaps ran out. Although I flew the One-Eleven for 7 years, I was never sure if it was the flap motors (hydraulic), the transmission shafts, or simply aerodynamic noise.
The brief grunts that are more typical of current types on the approach are probably also connected with flaps and/or slats selections; possibly the release of the associated brake mechanisms.
The characteristic whine of an A320-series (whether CFM or IAE-powered) when thrust is low or at idle, may well be from the two air-conditioning packs. It's similar to the loud noise you hear when they are running on the ramp with the engines shut down (using bleed air from the APU, of course). By the way, the series of loud grunts that follows the shutdown of the first engine of an A320 on arrival is from the hydraulic PTU (power transfer unit - the Americans neatly call them reversible motor-pumps), which is taking over the hydraulic system of the engine that has been shut down. When the second engine stops, the PTU loses its own hydraulic pressure and stops.
Now there's some thread-drift!