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Quite often I hear a kind of "howling" sound coming from airliners when I'm at a general aviation airport, which is located at 13nm from an international airport. Normally these airliners overfly the general aviation VOR at around 10.000ft and then keep descending while flying towards the international airport. Does anyone have any ideas about what could be the cause of such a weird noise? Could it be associated with any aerodynamic effect?