that would be...removing the sunroof via pulling back roof lining and replacing the seal. happened on mine. was a bit of a ball ache to do but easy enough once you got started! fixed it and any rattles along with it. the inner seal you need is about 0 quid form memory, nd looks like a glorified draught excluder!
oh also when you put the seal on, fill it full of black silicone! thatll make sure it wont leak again
it happens because the frames on the sunroofs warp over time