Can't know if your "rattle" is the same as mine. In case it is I thought I'd tell you how I got rid of mine.
At the base of the front springs there is a metal "base place" (roughly similar to a small cereal bowl). The purpose of the base plate is to keep the bottom of the spring stable. Tiny grains of sand, trapped in the base plate, would move under the end of the spring, upon a vehicle turn or going over a bump with a certain weighting/unweighting of the spring. Once the sand was under the spring and once the weight of the car came down on the spring- the sand would be instantly pulverized rattling the spring and any other parts nearby. Once I flushed the sand out of the base plate (about a year ago) the rattle never returned. Doesn't cost much and is a pretty simple fix. Good luck!
2012 Gran Turismo S