I'm getting that sound now too. I think it's because when I took the guages out, I lost one of the tiny guage screws. Since I wasn't using the stock guage anymore, I took one from that guage. I figure now the guage is loose, making that rattling noise.
yeah opening it up is the only way you going to really know what it is, chances are its just somthing loose vibrating and hitting somthing when the car vibrates.
And for your newest question the flux capacitor is from back to the future, it's what makes time travel possible. It wouldn't be behind you gauge cluster though, it would be right behind you where the rear seats are.