After trying everything listed in this post, and all the searching I could, its still there. Im totaly at a loss right now, and I cant think of a single thing that could be the cause.
Now its changed a litle though. On VERY small bumps (like the transistion from the road to a bridge) it thumps. On smooth pavement, it will sometimes sound like Im carrying a load of metal pipe in the trunk. It seriously sounds like the rear half of the car is gonna fall apart. Thing is, on some days and on the same roads, it is quiet as can be.
If anyone has anything else to add, I would really appreciate it. Right now Im considering getting some Dynamat clone and covering the entire trunk so I wont hear the noise anymore. Thank you to everyone that tried to help out so far.