Txipi84 and others -
I wanted to post here that I mentioned this noise to my dealer today while getting maintenance done (oil change, spark plugs, air filters) and they solved the mystery! There is a slatted cover to a vent system beneath the driver's seat. Coins can get down in there, and in my case the previous owner left me a dime and a penny in this area that were sliding around and you could hear them when decelerating / accelerating. Mystery solved, for me anyway.