Well if you are sure it is not the rear tire carrier (I have thought that there was something under rattleing only to find out it is the rear tire carrier), then I would get under there and grab and shak any of the suspension parts. If you can reproduce it by having someone jumping in it while you look around under it I'm sure you will be able to find it.
Don't forget to
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first then ask questions later! The loose nut behind the wheel