I have a metallic rattle from the right rear seatback area of my 2006 400h. The dealer (Lexus of Wilmington) has pulled apart the rear and packed insulation in the empty spaces, but it still rattles.
If someone is sitting on that seat, no rattle. If I fold down the right rear seat, no rattle.
The rattle doesn't have any relation to the road surface. It does seem to relate to speed, however. More noticeable at low speeds, or above 65-70mph.
Has anyone had the same problem? Any ideas to fix it?

 

Mine wasn't a metal sound, but I've found a couple rattles. One was the female end of the seat belt rubbing against something. When I reclined the seat or folded it down like you, it went away. The other one was the luggage cover in the back was rattling because it wasn't properly placed.

Hope that helps.

 

mine had that, my dealer fixed it, they said they "lubed the joints"