I have the same problems with my IS. A friend of mine was sitting next to me one day and as we drove on Westheimer/Houston, he said this is unacceptable noise from a luxury car!
Talked to the dealer about BUT he said"your expectations from this car is too high." he wants me to thiink that this is the way this car should be.
Did you also notice that the headlights don't shed long enough? Same dealer said nothing wrong with the lights(we calibrated them, etc.) then another time he admitted that when he drove a loaner IS in the night found the same problem with lights!) Yet no solution. I am not sure if we've incompetent dealers or what?
Please keep us posted as you try to mitigate/investigate this rattling/noise problem.