It was cold this weekend, so I did some fooling around...
It's not the actual panel on my truck that's rattling. ?If I hit the panel, it rattles, but it's louder if I hit back near the light.
If I open the sunroof's shade, the noise is just about completely gone.
I'm thinking it's the power wire for the light, rattling up and down, maybe against the roof or something. ?With the sunroof shade pulled back, maybe it goes between the wire and the metal of the roof, dampening the noise?
When I have a few more minutes, I'll try to take down the light and see what's going on.