Mine were AirPods Pro, incidentally there’s quite a lot of reports of this phenomenon with them (which might be coincidence of course considering their popularity).
I doubt it's the audio they emit, as they don't really emit any audio (when only cancelling noise). The "trick" with ANC is that it cancels out the sound wave that would otherwise be noise.
One of the theories around tinnitus is that some of the auditory nerves become "overexcited", causing them to fire more frequently, which essentially leads to your brain "amplifying" the sound, and the tinnitus is a byproduct of this amplification. Much like speakers will "hiss" if you turn on the amplifier without any music playing. The underlying reason for the amplification happening in the first place is usually mild hearing loss.
That also somewhat explains why (my) tinnitus is usually worse in quiet surroundings (again back to the amplifier analogy).
in any case, that's how my MD described it to me. I have mild tinnitus, probably caused by years of riding a motorbike without earplugs in.
so are low zinc, cysteine and histidine levels a factor for tinnitus?