I very much doubt that the popping is anything dangerous. However, if you feel your Kindle getting very warm or hot, that could be a problem.
Please PLEASE everyone talk to Kindle Customer Service through the Help portal on the Amazon site. Like I said, the more they hear about this problem the more motivated they will be to fix it.
At this point, it's difficult to say what exactly is causing the problem. Look at the questions I've asked in earlier posts and see if you can answer them. I may be able to tell you more about what's causing the problem if I know more about it... I don't own a KF so I can't test it for myself.
[EDIT] A Q&A thread on the XDA Developers site, which is full of very smart people, seems to have come to the conclusion that this is a bug with the speaker software when the speakers are in mid-to-high volume range. Try turning them down or off, or using headphones, and see if that helps. It's also been reported that turning off the "Dolby Digital" option gets rid of it. If it's just the software, there's no point in sending it back for another one, as that new one will have the same software. But a software bug is fixable with an update, and Amazon is very good at fixing these sorts of issues when motivated by customer pressure.
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