I live direct below one of the fix point (SONER) used on approach to Oslo Airport (OSL) from the south and this whoop is a familiar sound (and an easy way to pick out the single aisle Airbuses if you are not looking). Usually the A32Xs then have a ground speed of around 220 knots, and this is the speed the flaps position 1 is usually selected. Flap position 1 means the slat only is extended (max speed for 1+F is 215 kt), and I suspect it is the air that start to rush through the opening between the slat and the wing that makes this sound. A very short time after this whoop the engine start to spool up a tiny bit (from idle to approach idle), and the sound of this spool up reveals if the engines are CFM56 or IAE V2500. It is indeed still possible to "spot" planes only by sound, and with sites like flightradar24.com you get the definitive answer to what you hear almost instantly without bins
