engine foundation rattle is another thing, if foundation is bad, you can feel the vibration in the whole car itself as the engine vibrates, but as far as rattling "sound" is concerned you said it is coming from the trunk, so before going ahead with anything else, i think you need to diagnose better where exactly the sound is coming from. take a friend with you who can actually record a video and keep an eye when the rattling sound comes that where is it coming from... i mean is it from inside the cabin itself, or something underneath the car, etc. only then you'd be able to diagnose better and solve the issue.