You might try taking a can of compressed air and focusing it in the cracks. . .it's possible there's a crumb or something that's got in and is causing the problem. . . .but I wouldn't go any farther than that if it's still under warranty.
In general they don't 'fix' Kindles that are returned -- at least, maybe they do, but that's not the one you get back. And it's better this way! If you have a problem under warranty, they send you a replacement immediately and you send the other one back after it comes. . .that way you're never without a device.
