i have the the exact same problem. Genius Bar doesn't know. First Genius Bar visit was a ios wipe and reload. Didn't make a difference.
My first iPad Air 2 gold 128GB makes a random Pop sound. I also get random beeps and hiss sounds in certain games. I did an exchange at the Apple Store for another gold 128GB air 2. The replacement does the same pop sound. The replacement was worse with more random beeps and hisses when playing certain games. Both air 2 are on iOS 8.1 And both were setup as new Device. Not from backup.
I Decided to fire up my iPad 3 which is on iOS 8.1. Used it the same was as both gold air 2 iPads. No random sounds, no beeps or hisses, no random pops from speakers.
AFter returning the second iPad air 2, I decided to give it one more try And mix it up this time. I got a black 64GB air 2. surprise, it does the same thing. uncortunately I will be returning this one as well. It's too bad because it is a really nice device. no one knows if it's hardware and software. I don't want to be stuck with the device if it's a hardware issue. I might give it another try if apple confirms the issue and fixes it in 8.1.1.
tthe guy at the Apple Store said they haven't had one complaint about the new iPad air 2. I don't know if it's true or not but I find it hard to believe that they haven't had returns for the same issue.