"Belly Pop"

I love mysteries of medicine (okay, so only ones that are completely not harmful or painful).  Have any of you heard this one?

You're sitting there nice and quiet either trolling some Mommy blog or discussion board or Pinterest...whatever, and baby kicks.  *CLICK*  You hear a tiny noise that sounds something like a knuckle popping or a computer mouse click.  Did that seriously just come from your stomach??  Since when were there joints around there to pop?

No, you're not crazy.  Yes, that noise did just happen (it may have happened a few times in a row even).  No, there is no medical consensus currently out there to be found though plenty of Mommy boards have made various suggestions and guesses (and thrown out their midwives/OB's best guesses also).  My personal best answer is the ligament popping one (I think it meshes nicely with my "new stretch mark" theory LOL).

I had these noises a lot with my pregnancy for Alyssa and last night I got the first "belly pop" for this guy, followed directly by a second.  I love this click because it is just so bizarre and yet harmless.  It isn't my hips/pelvis like some theorize since it is the top of my stomach...it is too far out in front of me to be my ribs so the only bones involved would possibly be the foot or butt of my unborn child.  (Good try though folks.)

Anyways...I'm interested in theories just for fun but I don't require a medical opinion or answer from the internet masses.  Feel free to post your Belly Pop experiences or theories!