I just noticed this yesterday, on my 2 week old iPad. It happened once when opening the cover and entering the passcode, and once when pressing and holding the center button. I could not just reproduce it. It just happens. That's what brings me to the forum this morning. I can see that I'm not the only one experiencing the issue.
Mine actually sounded like a single popping noise.
Not sure what to do. There are no other problems with the iPad, and I don't want to trade for one that might not have the issue, but might have some other issues.
So many issues that these things can have.
Do we know whether it is a hardware or software problem? It seems like if it was a software problem, everyone would have it. Perhaps static discharge? I live in a dry environment, and it is winter.