allright, it might sound dumb but i've paid attention to this characteristic of the 757. When it is on approach you can hear the engines working normally, but when it passes right above you suddenly this noise changes abruptly, if you have hear it you may know what i'm talking about. After that, the sound again changes to the "normnal" one.
I want to know why specially this aircraft has this feature. Thanx!
Felix L Williams