Dimension E310 Rattles

I have a user with an E310. Great little machine, but he tells me that every so often it rattles so badly that it is distracting and aggrivating. I open the case to make sure no loose parts were floating around and that the fan was secured correctly and tightly. As i expected, everything looked fine.

Does anyone have any suggestions as to what might be causing the rattle and/or how to fix it? Thanks!