Odd sound, 2008 Sedan

[MUSKOKA800]

Perhaps some of you have noticed this annoyance.
My sedan has recently developed an odd sound which I cannot pinpoint.
It sounds like a marble/bearing is rolling back and forth as I excellerate and brake.
I've searched all comparments inside the car. Under the seats, within the seat tracks, even under the hood. All to no avail.
I'm left wondering if something is rolling about inside the front passenger side door but really would rather not pull the door liner for fear of damaging the attaching clips. Besides, unless something has broken away from the door speaker, power window, power lock or latching mechanisms nothing foreign could find it's way into that cavity.

Any thoughts from the esteamed membership???