It's most likely the bottom of the door panel rubbing against the plastic sill. Typically it becomes more pronounced as the temps fall, at least that's when I notice it. I've applied a vinyl conditioner (MeGuiars product) and that worked for a few weeks. Some have even applied cross-country ski wax!

It's been a topic on this board since I've been here, so for more ideas try the search function here and see what else you can come up with. With 16k miles, I can't imagine the door pieces being loose. It's more of an inherent issue with the 126 chassis.