Short Answer: ------------- The sound is caused by airflow across holes on the underside of the wings of certain aircraft. Long Answer: ------------ I too have noted the sound phenomenon that you describe. [This video][1] has a great example between 0:26-52 (link starts playback at 0:26 where the noise starts): http://youtu.be/8g0TWFUDDtE?t=26 I will readily admit that this answer is mostly just a compilation of what I have learned researching this. Your question intrigued me and I've enjoyed learning about it! As I discovered as I started researching this, many others are familiar with the noise as well. [This article on Airliners.net][2] had a lot of discussion about it, starting with a lot of interesting—but ultimately flawed—theories. But, that thread led me to the answer to your question, which is: the sound is caused by the "circular opening used to equalise the pressure in wing fuel tanks", according to [Euractiv.com][3] [This article on AirportWatch][4] further explains: > The Airbus 320 series of aircraft, [...] have been known for many years > [...] to have a particularly irritating high pitched > whine. This is caused by air rushing across the under-surface of the > wing, where there are Fuel Over Pressure Protector (FOPP) cavities. > This generates noise, in the same way as blowing air over the mouth of > a bottle. Every A320 series aircraft emits a signature howling noise > while approaching to land. It is heard most when the plane is > travelling at around 160 knots, and the frequency is around 500-600Hz, > which is close to peak sensitivity of the human ear. That being said, there is apparently a fix to that problem, as both the aforementioned links describe. As [Euractiv.com][3] explains, the fix is a vortex generator: > a 5-cm triangular piece of aluminium sheet metal upstream of the two > vents on each wing, in order to divert the air flow and stop the > whistle. A decade of research went into the vortex generator, which > stems from efforts to mask a similar whistle produced when air blows > over the gun ports of warplanes. This image shows both the FOPP hole, and the VG that softens the sound: [![A320 FOPP and VG*Image Courtesy of Lufthansa*][5]][5][Image Courtesy of Lufthansa] Read the articles linked, interesting stuff therein! [1]: https://youtu.be/8g0TWFUDDtE?t=26 [2]: http://www.airliners.net/aviation-forums/tech_ops/read.main/321412/ [3]: http://www.euractiv.com/sections/aviation/small-beautiful-how-tiny-device-cuts-aircraft-noise-303505 [4]: http://http://www.airportwatch.org.uk/2014/11/lufthansa-retrofitting-a320-family-with-simple-inexpensive-noise-reducing-device-to-stop-the-airbus-whine/ [5]: https://i.sstatic.net/Gu0fM.jpg