I just opened an AEK II a couple of hours ago. After seeing the inside, I'm not sure what could be rattling. Is it from the area of the height adjustment? Is your height adjustment functioning or could a spring have come loose?
The case halves fit together quite snugly, there is one screw and lots of snaps. The keyboard unit itself is bolted together and snaps into the plastic casing. Perhaps one of your interior screws that holds the keyboard mechanism itself together is loosening?
Either way, you're going to have to take something apart to find the source of the rattle, I think.