Clonking noise in boot

I have a strange clonking noise at the boot end when I pull away, it sounds like something rolling backwards then forwards. It dose it only when I pull away. Have sat in the boot with my hubby stopping and then pulling away to try to see if anything is loose but can`t find anything. the spare wheel is tight and the jack is secure. It seems to be more to the passanger side the exaust isn`t loose either. Any suggestions as it`s driveing me mad