Had the dreaded pax seat rattle..took in 3 times..no luck. Finally, President of Dealership exchanged his SC430 pax seat for mine (Hows that for service); eliminated 85% of seat rattle. Then, I moved seat forward so as not to touch rear seat plus, went to full lumbar...eliminated 10% more. Will wait till 5000 service (now have 4000) for final 5%. Every now and then, get that "tinnie" buzz... I chalk it up to the great a/c inside and the BAD Texas 100 plus temp outside..working on the metals. Also, I suspect the hard, hard run-flats...which shake up the car at every pothole/bump... are probably exacerbating the "problem". I'm lucky to have a VERY supporting dealership. (I get a LS430 as a loaner!!)..so I know they will work with me to correct anything correctable.

 

My guess is that if you wanted an LS430, you would have bought one. Have the dealer check the wiring harnes inside the seat- it rattles against the hard plastic of the seat over bumps- all they have to do is tie the wires together with one of those plastic ties and your rattle should stop.
Attentive...IMO Lexus should not have all these different rattles with no idea how to fix them, 1st year car or not.