Well, I've heard this for a while and was wondering what it was. Most of the time I go in reverse on pavement(only seems to happen on pavement) I hear a noise comming from the back of the car that sounds like I have a pipe that fell on one end and is going up against the pavement as I go backwards. I know that's not the case, but that's exactly what it sounds like. Any idea what this would be? I've looked under the car and I see nothing that would be actually touching the ground, or even that close really. It's just a little disturbing...because even though I know it's not I get this feeling that makes me cringe every time I hear it thinking it is the exhaust scratching the ground.