The left speaker on my Fairbuds XL has issues where the noise cancelling is faulty. It occasionally creates loud static noises when the headphones are moved, as well as diminished quality of noise cancelling in general, always on the left speaker.
If you haven’t already, try removing the cover and checking for dust or hair near the ANC mic. It may not help, but it’s easy enough to be worth a try.
Otherwise I have no further ideas to help, but wish you good luck with finding a fix.