I am new to AR15's myself. My 3 week old Dissapator's handguards rattle a little as well. I thought it was normal, but would like to hear as well.
By the way, I have been to the range both of the last two weekends with the Dissipator and I love it. I have sighted it in and am shooting acceptable groups at 100 yards with the stock A2 sights. I have a long way to go (sight picture, trigger, breathing, etc.) to get my groups were I want them to be.
Good luck with the Dissipator!