This may or may not be relevant, but my IS300h had a squeak coming from the back of the car, especially when going over small sharp bumps. I suspected the rear head restraints, so removed them for a few days but it came back. The car went back under warranty for a week and after a few road tests it had a new shock absorber fitted on the rear, but that did not fix it. Eventually after going back twice more they removed the rear window and re fitted it, and it cured the problem, apparently there was a small area that was not sealed correctly and it squeaked going over a bump.
Hope you do get it resolved, squeaks and rattles are frustrating.
Peter