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Since I have flown in aicraft as a kid, there was a sound audible when the aircraft is on ground (before taking off or after landing) which is similar to the sound a DJ does when he scratches the disks.
Sorry for the wird example, but I do not know how to explain it better. It is like the scratching of a disk and the sound starts at a higher pitch and it ends at a lower pitch. I am really interested in finding out what it is and why it happens, since I have never been able to find out how it is produced.