Finally got rid of the rattle, changed the factory rear tokicos with kyb shocks and all rear rattles are gone.
I have the same rattle as you. Do you have a link or a part number for those KYB shocks? And have the rattle came back since the shock replacement? My warranty is up, so i'll probably end up doing this myself.
I forgot to post my update. I got my car back and I thought it was just a loose sub housing but the dealership ended up insulating the entire back area. The sound system sounds so much better with the insulation and there are no more rattles!!! I'm very happy with the results.