2006 XC70 - I can hear something rolling around in the back of the car
 

Hello! The past couple of weeks, every time I go up a hill, down a hill, over a bump, I hear what sounds like something small, heavy and round (bolt?) rolling around from the back of the car. I have checked all of the side panels, under the trunk where the spare tire is held (I've taken the spare tire out) and can't find anything. Is there another space something could be loose and rolling around?

Thank you in advance for any ideas / fixes!!

Model: 2006 XC70