Something loose in MBP 13

[Two-Four]

I've been noticing that in the bottom right-hand corner of my MPB there is something that seems loose. When I tilt the computer front to back, or pick it up to move somewhere else I can hear something slide back and forth. I thought it might be a loose screw or something so I opened it up but didn't see anything. I've only noticed it when the computer is on and I know the harddrive has moveable parts, so is this normal?

 

[miles01110]

It's probably the sudden motion sensor. A part sliding around only when the computer is on makes no sense.