Loud rattle in rear of Prius c 2014

I have owned my prius c for two months now....I have had a metal rattle that was driving me crazy....it sounded like metal pans banging each other...I went through the back seats and put them up and down but the rattle was as loud as ever....I even renamed my car Rattle Trap....I shook the back doors and had my daughter ride with me trying to discover where the rattle was coming from....I could not figure it out and then my daughter said, "Mom, check your license plate frame" I had put on new Hawaii frames in the front and back and thought could it be the source of the rattle?? I took the back license plate frame off and re-tightened the screws.....I got in the car and took it over a bumpy street and guess what?? NO RATTLE.....if you are having a metal sounding rattle near the back area of your prius check the license plate frame....can't hurt....