I haven't noticed the marble in mine. (I'm at 11k now)
Could this be what you guys are hearing....

At about 3k, I found a plastic coated nut (as in nuts & bolts) rolling around at my feet. It had to have come from under the dash.
The nut was not your everyday nut. It had some sort of extra groove running across it's face
I have no idea what it was for.