Why do airplanes make a humming sound when airborne while the engines (when on the ground) make a steady sound?

Question

I know experience-based questions are a bit tricky on this site but isn't every theory based on common experience?
Every time I hear an air jetliner flying over at a high altitude it seems the engines are sort of humming.  The sound isn't steady in time but instead little variations are to hear. They are not making the steady sounds of the engines you can hear when these are working on the ground.
What's the cause of this? The clouds?

niels nielsen · Answer

The humming sound is radiated by the rotating fan disc at the front of the engine. The outer periphery of the fan disc is traveling through the air at a speed close to that of sound, which causes the hum. The closer the blade tips approach the speed of sound, the louder the humming becomes.

Answered by niels nielsen on February 10, 2021

akhmeteli · Answer

At least some differences are caused by the Doppler effect, as the direction to the moving plane changes with time.

Why do airplanes make a humming sound when airborne while the engines (when on the ground) make a steady sound?

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