The last time it went in for the rattle I insisted that it be rectified once and for all as this was the fourth time it's been back, I told them that they could keep it as long as necessary to do a thorough job and not just a botch up. They had it for a few days to remove the roof lining take out the sunroof from its frame and install foam deadening strips. The conclusion was that the frame is made of steel but the sun roof cassette is aluminium and they were expanding and contracting at different rates, how true this is I have no idea. When I got it back it was still rattling for next couple of days but then it stopped, coincidently just as the weather got colder, whether it will start again when the weather warms up is anyone's guess. This is a recording of the rattle...
Lexus noise.mp3
I definitely got the car that was built on a Friday afternoon. I hope you get yours sorted quicker than me, good luck.