I found mine on the rough road between our country house and the highway. I got home and opened the trunk. I removed the layers of carpet to get to the spare tire and jack. I found the left side of the cardboard was loose and sticking above where it should be seated.

The hand fastener that holds that thick board down was not tight. I removed that cover to make sure the jack and spare was tight which it was. I carefully reseated the cover making sure the two parts of the jack were sticking up though the cover cutouts. I replaced the hand fastener and tightened it moderately. I put the two carpet covers down and now no more noise.

My noise was a very low level rubbing or slight rattling sound. On a rough road my wife and I both could barely hear it. The noise only developed a few days earlier. Could be the rough 2 miles road to the house had loosened the hold down bolt over the last 9 months of ownership.