Lately, I was watching fighter jets (F-16s and F-35s) from very close (fascinating!) and heard for the first time the "whistling noise" when they were landing and taking off. I expect it has something to do with the compressor of the fighter jets, but I am not sure because I found some sources on the Internet and they all actually say something different:
I was wondering if anyone could explain with some good sources why this "whistling noise" happens and maybe its functionality?
Turbines have a characteristic whining sound that is most likely what you are hearing. You can hear the same effect on a turbocharged car when the turbine spools up. The sound itself isn't functional, it is simply a byproduct of the compressor working as it normally does.
When the plane has its gear down and its flaps deployed, the airflow over the open landing gear wells and across the gear struts and whatnot can produce a distinct whistling noise that is strongly modulated by speed and angle of attack.
Even small planes like the Piper Cub "whistle while they work" and you can hear this when a Cub pilot cuts power on approach. The engine noise dies off but the whistling persists.
The most famous whistler is probably the P-51. there are many youtube videos of P-51's doing low passes and steep pull-ups at high speed, and when the AoA is just right they make a very loud and creepy-sounding whistle noise.
