I had a loose kind of clattering noise coming from under my car and I honestly thought I had tranny problems, but what it ended up being is the metal retainers for the rubber exhaust pipe mounts at the trans bracket location. They rattle when driving the car and it gets worst the colder it gets. I guess the metal contracts when colder. There is a TSB for this. the fix is to tighten two large hose clamps around the assemby that the hanger pins from the exhaust pipe assembly fit into. For the 4 bucks it cost me for the clamps i did this myself and the rattle disappeared, I could'nt believe it but it worked and I would never of thought to suspect this as the cause of the noise. I not saying this is your issue, but check it out, it just might be.