Are you talking about the stock itself, loose on the tube? Electrical tape inside the slider will help. If you're talking about an internal rattle, the buffer has sliding weights that clatter around - normal.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek