If it is metallic sounding, and its a tapping noise...does it do this while applying the brakes?

I would still double check the sunroof motor and cover, it may be rattling when you hit bumps.

I forget, did you check the mount for the transmission yet? The rubber could have worn away and is causing it to basically bounce when you hit a bump.

How about the actual glass, is it spreading away from the frame at all? Could the cable for the auto-hatch release have come loose?

I'd pull out the trunk carpet and bounce the rear of the car up and down to see if it makes the same noise.

 

Check that the emergency brake cables aren't tapping against the rear suspension arm. I think this is happening on my car over bumps.

 

sm: I also have that same rattle, after it's run a bit it's gone.

 

Ahh yes! The parking brake cable. How is the adjustment when this happens? I pull my lever above the seat for it to work...I don't hear anything, but I don't hit bumps too often at speed. Is this why the cable would tap, or are there mounts/fasteners which come loose over time?