Rattling in trunk


I have a two year old Mazda 3 with about 32,000 miles. Today I noticed a rattling noise coming from the back of the car. It sounds like there is a tool box in the trunk and every time I go over a bump everything in the tool box jumps around. I emptied my trunk - there was not much in it - and checked the spare and the jack and accompanying tools and all is snug. What could be making this noise and do I need to take it in for service?