I always thought this rattle was my roof rack until I took it off, turns out it's the sunroof. After extensive digging around on this forum here's what I've found on sunroof rattles:
1. rod silence fix - put a rubber hose or zipties around one of the rods up there (this is not your rattle so it most likely won't help you)
2. sunroof guide rail grease - tilt your sunroof up, remove the two black plastic covers that are on one side covering the mechanism. You'll see the guide track for the glass, it's a zig zaggy shaped guide that a pin slides in. The rattle you and I have is from the pin being able to move vertically in that guide track. Some people mention tapping the tilt button to preload the pin against the top of the guide rail and preventing the rattle temporarily. Lots of discussion on Audi forums about panoramic roofs rattling and they simple grease the guide with some super heavy grease. I haven't looked around for what grease is best to use here but something very thick to help cushion the pin.
3. stuff it fix - The opposite of preloading the rear up with the tilt function is to close something, rags, rubber vacuum cap, etc. under the glass to immobilize it. This is what I'm running with currently for the winter but will make opening the glass in the summer a pain.
4. gummi pflege stift - Some people will say that the track is supposed to be this loose and the issue is really that your rubber is not sealing to the roof to hold the glass in place properly. The rubber is not available separate from the glass so I've started a regimen of the best rubber treatment that there is out there (also from german car days, rattling hard top convertible fix) this stuff called "gummi pflege stift" to attempt to restore the sunroof rubber back to it's original state. I don't think this has helped my rattle at all, might have been too far gone to save, but it does make you feel good when restoring all the door seals on your entire rig.
5. the dealer favorite - "your guide track is worn out and is only available as an assembly" remove and replace entire sunroof mechanism.
(picture from another vehicle, ignore the circles, just for reference of what the guide track looks kind of like)