Had the same thing. If you do a search for "creaking" you will find many posts and a TSB. I took a 1700 mile road trip in my brand new 2006 sport and nothing even whispered from it, then I park it in the garage for about 3 days and drive it to the grocery store and it sounds like the passenger rear strut tower is about to come through the inside!
Try reseating the seat on the anchor or lubing the bar with tape or whatever. It should go away.