I would say that code58 nailed this issue. For the most part this method works. I think the back door bounces back a bit (even though it seems closed) if you treat it like any other car door. That is to say if you just swing and call it done, it will rattle. I can't really tell what exactly is happening to cause this rather subtle effect, but code58's fix seems to work for me. (07 XLT SCab)

 

I dont know but I checked the back door and it latches tight.When it is closed and the front door is open there is no give it is closed tight. I think when the driver door is latched it must be worn some we I put the felt tips between the doors it stop rattleing. I hope this will work for someone else.