Hopefully the fix for yours was a simple one! I tried tightening the screws differently and had no luck as my cracking/popping sound was coming from around the right palm rest rather than along the edge. Check on those screws often though, my buddy has lost some of his due to them backing out...seems like they walk easily on these newer machines.
I think my cracking/popping noise was due to the battery not being glued in correctly. Maybe too little glue? I took mine in to the Genius Bar 4 times before a tech acknowledged that the sound was there. Case had been tightened/loosened/reseated/etc all the other times. Finally they said they would replace the top case and the bottom case for good measure. Got the laptop back, and it feels more solid than before with no noise. The reason why I think it was the battery is because the new top case comes with a new battery glued to it as 1 part (also includes the keyboard apparently).
Hope this info helps someone. Be persistent, get them to replace the top case. $2000 gives you the right. If they're gonna brag about manufacturing processes and precision/perfection...then we get to demand it when we pay for it.