Thank you NC211,
I've check the brackets, those rubbers are fine. In fact, most of the time, the noise seems to come from the steeringwheel's neck. It sounds like when you have two block of dry hard wood in your glove box. The sound is much clearer when I drive through an old brick road or railroads- slowly .
When the car stop, key's out, I move the steeringwheel shortly but rapidly, it make a similar sound, not exactly the same though.
The noise is not that bad, but, as you know Lexus LS is super quiet, it does bother a bit.