If you lift up all the carpeting and such and look at the very rear of the car on the two sides, you'll see that there are passages that lead down the rear side of the car (right behind where the rear side markers are). At least on my 2+2 there are. When I ripped out all my stuff to put in a box, I noticed the hole (it's also where you access the antenna motor). I dropped a wrench in there and had a similar sound. Worth a look!