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At first I thought the hiss / whine was caused by the battery being under load. I also read someone else speculate it was caused by the CPU being used. So, I did an experiment.
I took my Nook Touch and did a firmware update. That uses both the battery and the CPU during the entire update. The whole time I held my Nook close to my ear.
It turns out it only made the hiss / whine sound when the screen was updating. If the screen wasn't actively updating or drawing, it was silent. So it would appear the hiss / whine sound is caused by the Pearl e-ink screen updating and drawing.
If that is indeed the case, any other e-reader with a Pearl screen would make the same sound while the screen is updating and drawing.
This would also explain why my old Kobo Wifi, which uses a different screen, doesn't make this sound.
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