Hmm, I tried shaking it as you have down but to no avail and apple care is as good as oblivious to acknowledging the issue. I may just have to visit a retain store much to my dismay as they tend to mistreat your current device then if they do issue a replacement and that has an issue of some sort, they are reluctant and often refuse to issue another. It's a headache inducing process I would rather avoid but the rattle is audible in a quiet setting so it seems inevitable. Hopefully you were able to solve your rattling indefinitely without having to haggle with the Apple Store at a later date. Another 6s plus device that my sister owns makes a similar rattle so I'm not certain if it's a loose component or simply a faulty Taptic Engine.