Thought I should provide and update....
I have watched a bunch more movies and it seems to only do it on certain movies (or at least I notice it) on those movies where the all the speakers are mainly quiet and theres bass going to the sub. Again, each time I put some weight on it, it either stops this completely or greatly minimizes the rattle. For the first few weeks, I placed a very big/wide 10 pound book on top of the sub box and that helped a LITTLE too.
I have spoken with HSU staff (as stated) and rather than try to sell me a new sub or sell me a new enclosure, they suggested a brace in the sub. I finally got around to doing this yesterday. I added the brace as suggested with Gorrilla glue. I was gonna let it dry for 24hrs but noticed after 5 hours that brace was rock solid in there so I let it sit for another 2hrs and watched our first movie on it. I did not hear a rattle but thats not to say that it completely worked because this rattle only seems to happen in about 10%-15% of the movies we watch. I will test it out for another week or two and post my results.
PS....when I pulled the woofer out, I could not believe the size and weight of this thing.
Also, HSU's staff is incredible, I cant believe the support I got from these guys 5 years later.