Crackling noise on left speaker when walking or in a moving vehicle

To determine a mechanical from electronic noise, leave headphone power off. If this noise goes away then it's electronics otherwise headphone flexing is creating noise. If it's electronic noise then review sidebar questions on disassembly to find loose or broken wiring.

Ok, so it seems that the noise is there even if it's off, but almost unnoticeable. So the ANC is picking it up and amplifying it?

My understanding of automatic nose cancellation came from aircraft headphones, attenuating ambient noise like very loud engines to enable better voice communications. A specific range is determined that's reduced wth an equal and opposite noise signals to cancel it, leaving clearer voice comms. For music or listening to podcasts surrounded by outdoor noise, a specific range is determined to produce an equal but opposite noise to cancel it. Wth plastic and metal flex points, examine where you think nose is generated by flexing to determine if a small amount of lube can help eliminate it. Something like silicone grease or spray, using a Q-tip and working it in until lube reduces or eliminates flex noise. It's debatable whether or not noise is amplified by the microphones since noise cancellation requires microphones to listen for ambient noise in order for electronics to generate equal but negative noise signals; the basis of noise cancellation.

Yes it should produce the inverse sound wave to cancel it. Usually low frequency noises with the same pathern.

I tested it again at home where it's silent and I couldn't hear the noise while it was off... Only when it's on and with ANC or transparency mode on. Maybe I though I heard something while trying to listen to it and looking for that pathern (weird stuff)

So maybe it's eletrical at the end?

Were these headphones dropped onto a hard surface? Jarring anything loose in each side may loosen wiring connections, break plastic and allow loose parts to be picked by microphones as part of noise cancellation except the noise is momentary with unwanted feedback.