TranquilTurbulence , 6 hours ago (edited 6 hours ago)

The noise cancelling headphones are listening to noise coming from the outside and cancels them out. When the source of the sound is in your mouth (see also: bone conduction), the headphones can’t hear or cancel that sound.

The headphones should still cancel the background noise as usual, but they can’t cancel whatever noise is coming from your mouth. I wonder if this mismatch could result in some noise getting through?

If you’re wearing buds, the seal could become inadequate while your jaw is moving, so that could also explain some of the noise.