I bought some 1/2 inch wide Velcro adhesive tape (it comes in two strips -- one has the hooks, the other with the loops) and used the loops strip (the softer of the two). I taped it to the back of my license plate frame (I have heavy chrome plates, see signature, so they were really rattling when ever I closed the liftgate.) It provides the perfect cushioning to keep the plate and frame from slamming into the body, thereby causing less/no noise and eliminating the potential for scratches beneath the plate due to the repeated impacts.

I trimmed carefully around the edges with an Xacto blade so the Velcro loop material extends right to the edge but isn't visible when viewing the license plate or rear of the vehicle.

- Shannon (Geeky, Wild Texan)