I have a similar sound in my car. I was told by Don at DMC Garden Grove that there are bearings in the rear of the car somewhere and the bearing a little bit smaller than the casing that it is in. He believes that the sound is being made by that bearing(s) that shift around. He actually did a much better job of explaining it than I just did. I just did a poor job of listening. Has anyone else heard of this before? -Christian