Finally found a way to reproduce this pop sound.
Play any music using the music app.
Go to settings -> music -> equaliser
While the music is playing, change to any equaliser preset like night and it will make speakers pop after tapping.
So it is clearly software / sound driver issue. Sometimes the popping goes away for a week and then comes back.
Both my Air 2's do it. Shameful QC testing at Apple lately.
I think every iPad will be having pops or cracking. People just don't notice it. But it is super annoying none the less.
That worked for me but doesn't make as loud/sharp a pop as the real problem.