It might be the piece of plastic behind the door handle on the inside of the car.
I also noticed the piece of plastic behind the stationary door handle (that you use to shut the door from the inside) is also loose.
I tightened these both down - but I also hear another noise coming from the inside of the door. The noise seems to disapear once I lower the windows.
I gave up looking for it though. I am going to live with it.