I Have this issue as well. I can often get the iPad Air 2 to make the loud popping sound after being in a game then going out and into email.
And if there is a new message to download and email tries to play a beep just as email opens it will make a pop sound.
I went to the trouble of opening a case but Apple are refusing to acknowledge that this is a problem. I purchased two iPad Air 2s, one for myself and one for my wife. Both have this annoying loud popping sound.
Apple support seems locked into following scripted answers and wants me to erase and set up the iPads as new. Um, hello Apple, they are brand new and both do it.
So disapointing that they won't respond or acknowledge this as a real issue.