yeah when you are running a decent amount of power to a trunk... you ARE going to have rattle most of the time. if i were you, id buy a roll of sound deadening material (dynamat, RAAM mat, second skin). do your whole trunk and a few layers on the trunk lid. that will help and also add some weight to it.

i wouldn't worry about it TOO much. you shouldn't be concerned about what it sounds like on the outside. the inside is what really counts.