I'm kind of wondering if this might be happening to others. Here's what I mean. Under the battery cover and under the back cover is a bar that holds the mechanism for locking the battery cover and battery, and the same bar holds the screws for the large back cover. The other day, I was just checking everything out, blowing out dust and such, and found that this bar was very loose. Every screw on it was loose. I tightened it up. No real problem here, just thought I'd let everyone know if they want to check there MacBook or MacBook Pro. Here's the bar that I'm talking about. Squared with red are the screws. Here's a closer shot of the left side as you are looking at the bottom. Here's a closer shot of the right side as you are looking at the bottom.