I've had the random pop/crack effect now over the 5 days I've had my Ipad Air 2 (week 40 build). Triggered by opening an app or putting a cursor ready to type in a search bar. I have been able to force the effect by just repeatedly entering and deleting a cursor in a search bar on various apps (for eg if you have the new Ebay app and touch the search bar and then tap to one side to remove the cursor and back to the search bar repeatedly you should be able to trigger the effect). There are some similar examples of this on YouTube. But of course in day to day use it just happens randomly. From my experience the loudness of the noise remains the same regardless of whether you have the Air 2 on mute or sound activated. Also, in my case anyway it does appear to occur more frequently at higher charge levels(above 80%).
I've been through the troubleshooting with tech support, including a full factory restore. So I guess the next step is to get it replaced and hope the replacement doesn't do the same thing. I've wasted too much time already with this issue.
But it would be nice if Apple by this time at least worked out whether it was hardware or software related or even acknowledge the issue(one of the techs I spoke to said that although he hadn't come across it before, the symptom did suggest hardware related).