Engine Clanking on Cold Start

Hello all,
I have a '98 GT w/ 47,000 Miles on it. I noticed over the past few winters that when the engine is cold and I start the car I get a clanking sound coming from the engine for no longer than 2 seconds. It almost sounds like a bunch of marbles bouncing around in the engine. The only time I hear the noise is on start up. The engine sounds and runs fine after I hear the noise. It only seems to do it in the winter....it doesn't do it in the summer. I am using 5w-30 Valvaline Durablend and a Motorcraft filter. Any Idea's on what this noise is and how to get rid of it? Any input would be much appreciated.
Thanks