I have the exact same problem. It pisses me off. I don't hear it on all roads though, only ones with a grainy or rough surface.
I'm able to pull the dome light behind the sunroof down out of the headliner by pulling firmly on one side until it comes down, and then the opposite side comes down as well. Once it's down it just dangles by the wiring which there seems to be about 5 inches of play or so. Maybe my dome light isn't quite as tight as yours. Try doing it in cooler weather if possible?
One thing is for sure the rattle isn't coming from anything in the dome light assembly itself. With my wife driving, I had it pulled down one time and was holding it real tight. I could still hear the rattle up somewhere in the roof and it wasn't coming from the light. Like you describe though, applying presure to the dome light (when it's in it's place of course) while driving makes the rattle go away.
I've only pulled it down twice, once while my wife was driving and once by myself with the car parked. I wasn't able to see/feel anything obvious in there causing the problem, but I think I would have better luck if I would have been in the backseat instead of the passenger seat the time I tried while the car was being driven. I may give it another shot sometime, but I think the headliner will need to be dropped to figure out the problem and I'm not sure I'm capable of handling that task by myself. I've already had it to the dealer a few times for this and they struck out (but I'm really wondering if they actually spent any time on it). It's a hassle to do, but I think I'm going to take it back again for them to look at it.
If I'm able to get a fix I'll let you know how. Good luck!