I returned three and they all popped... I finally returned mine for a refund instead of an exchange. I love the Speed: 2GB of RAM, 64 Bit of architecture, the A8X processor, and A8X mobile processor, but I believe there are issues with the conductivity within the device.
I also noticed, after reading Tech guides, that there are two versions of the iPad Air 2. One UI iPad5,3 & the other is UI iPad5,4. the Benchmark on the iPad5,4 is a bit higher than the iPad5,3. Everyone I know has an iPad5,3... The Tech reviewers all received an iPad5,4 to review.
This is not a software issue either. Any motherboard that makes a popping noise is not good, especially if it is located right where the Processor and RAM are.
I have heard folks trying to talk their self out of returning their device by saying it may be "settling" computers especially SSD driven computers do not have any moving parts that would "settle". They are soldered to a mother board along with the processors, RAM, Qualcom chip, etc... In other words there is nothing to "settle" or nowhere to "settle" to. I would strongly suggest returning yours if it is making any kind of popping noise. As terrible as it may be it is the only real way to resolve the issue. Contact Apple via http://www.apple.com/feedback/ipad.html and share your experience. Read the hundreds of Apple lovers expressing all of their efforts and remember that Apple's Flag Ship will soon have a fix...