The source of the sound is the speaker element of headphone. I thought that detail was obvious. A speaker reproduces any signal fed into it as to best of it's abilities. Acoustic recordings, sounds mimiking acoustic sounds, analogue or digital synthetic sounds, static noise... And even a digital pulse that goes from zero to maximum amplitude in one instant that is extremely rare or near impossible for even the most aggressive acoustic sounds. Acoustic or analogue noise is basically a sum of random frequencies all playing at once, while digital noise is a constant stream of random clean digital pulses.

Earbuds that aim to create a seal in order to isolate from external noise are dangerous in particular as there is nowhere for the sound waves to dissipate. Some parts are absorbed by the flesh of the ear canal but other parts become resonant waves that only add to the amplitude and hence the stress to the ear drum.

I had a pair of faulty ANC earbuds that would make digital pops. They weren't necessarily louder than the music playing but damn they hurt like an unklefucker. It was like pure spike of treble cutting through the ear straight into the brain. The type of sound our ears have never encountered naturally in all their years of evolution.