My new wife has a bone stock Mini Cooper she bought new in '07. It now has 63K miles. I noticed a light knocking/rattle sound from the back of the car that seems to track with suspension movement or bumps in the road and not road speed. I took the spare tire out and there is nothing loose in the trunk but the knocking sound persists. It is not a heavy, deep knock but a noise that seems out of place. Anyone have experience with this?