Hi
I have had my rx300sel for around 6 months and i too have a creaking noice while the car is warming up - it seems to be coming from the passenger side of the dashboard, either the air vent or from the air bag compartment.
More annoyingly i have a rattle from somewhere within the roof lining - maybe the sunroof, seems to be above the front passenger. I have to drive with the cover pulled across otherwise is drives me up the wall.
I spoke to my dealer about it but after very poor service from them over the years i don't have any faith in them fixing it. I have also lost faith with my next nearest dealer over problems with my previous car, an IS300.
Anything you find out about your rattle i woud find interesting.
I wish i could find a dealer that i had faith in, i am on my third lexus and even though i love my current car I sometimes think i might try a different brand next time simply because of my local dealers attitude.
Jon