I could hear a rattling under the vehicle and thought it was a loose muffler. I could hear something slightly rattling when I was idling, but could not locate it. Finally I got under the Jeep and pushed up on the heat shield. One side of it was no longer bolted up and it was resting on the muffler, causing it to rattle. We purchased epoxy and "glued" it back up. There is a recall on this, but not for the 4x4.