I had the same problem, adjusted the bumper stops and the rattle soon came back. I found a simple fix. The rattle is coming from the hatch latch. The top part is sort of "V" shaped and fits into the female slot of the bottom tail gate. Look on each side of the male "V" and you will see the paint is worn and has been rubbing the female slot. This is what causes the rattle/knock when you hit a bump. I took some 1/2" wide thick rubber tape, about 1/8 inch thick and put a piece on each side of the female "V". Have not a single rattle in months. Was driving me crazy also... hopes this works for everyone....