I have the same rattle. Sometimes drives me crazy but most of the time its unnoticeable. Usually if i hear it rattling I stop at the next chance i get, recline the seat a bit and sit in the seat to add pressure and its usually good after that. It seems like it only rattles in 1 position because when my girlfriend gets in the car and changes around the position it seems to rattle more once shes out of the car.
2012 Sterling Grey Ford Focus SEL Hatchback