During the operation of engines that emit loud noises such as lawnmowers and pressure washers, I've had similar experiences. With the lawnmower, the deafening roar reached both ears the moment the blade struck a stone. Using the pressure washer, it was highly unpleasant to hear the loud abnormal noise, possibly caused by splashes or wet components touching the microphone. I believe the abnormal noise from the lawnmower could be resolved with a software update.

In which ear(s)?

In my case, both.

Personally, the left ear.

Does it matter which side of the plane you are on or how close to the wing (engines) you are? I could imagine an audio difference between left and right if you were sitting at a window near an engine.

I did sit on both sides (one flight was next to wing) - did had same issue both times.

One thing that was common- I was sitting in a window seat.

Just got back from another flight, where my seat was in the middle of airplane (3 set of rows). No issues whatsoever. One thing I’ve noticed, if I press AirPods harder, I do start hearing hissing in my ears.

Seems pretty odd that all the reports are about the left ear.

with buds, whichever connects first (usually right) is the master, and the other bud connects to that one as the slave, instead of connecting directly to the source device. probably has something to do with that

Which would hint at a software issue. When it comes to ANC, both should behave identically