How do I open?

Yes, you can open them. You will need a Phillips #0 screwdriver to get them open, and a plastic card or flexible pick (or even a crochet hook) to get them back together. There are 2 screws on each side clamp (the parts labelled L and R), and three or four on each ear (under the ear cushions, which you can pull off by hand without damaging them). I would check the ears first for the rattling as there is more room inside them than anywhere else on the headphones. The procedure for getting the ear cushions back on requires a bit of patience - tell me if you find you need a video to demonstrate :)