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I'm assuming the sound is coming from the reader, not headphones, and the '600 does not have a built in speaker or beeper.
Googling around, it looks like the eInk display needs 15 volts to change what's displayed. With a 5.2 volt charging voltage, there's probably just a single 3.7 volt LiIon cell powering the reader, so there must be a DC-DC converter in there to step up the voltage. Typical (all?) converters like that use a transformer/inductor and depending on how well it's potted and how it's mounted, it certainly can be a source of audible noise.
Perhaps it's very near the display on some units and touching brings the screen into contact with the transformer and makes it audible.
Just my pre-coffee ramblings, please ignore.
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