My 12 month old IS rattled in the mornings too, it was traced to theh wires/drain pipes from the sunroof, moving behind the trim on the A post.
As to comments re German cars, I have had 3 VWs and 2 Audis, the all rattled .......badly, in fact the IS is the best car in the ratle stakes I have owned in 12 years. I think it is getting to the stage where a rattle free car is a rarety, trim materials get cheaper steel gets thinner, is it any wonder?