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.
US Army 1966-69, VFW Life Member, Retired Geek
I'll give it a try, if I don't like it is easy enough to remove. It's just with all the AR stuff on the market I figured someone had something a little more perminant. I got the AR bug right now and lovin it!. 3 years ago I would not have given you a plug nickel for one of the ugly SOBS now I'm hooked, what's up with that?
If the stock is excessively loose on the extension tube (Buffer Tube) There could be a chance that someone has put a Mil-Spec Extension Tube on the rifle and a commercial production stock. The Mil Spec Extensions Tubes are smaller in diameter than the common produced commercial model and require a mil spec stock. This happens a lot of times if someone built the guns from a DPMS or other manufacturers stripped lower by using miscellaneous parts. If it just has a slight wiggle or rattle that is common on most all collapsible stocks no mater who the manufacturer. As far as any noise caused by the buffer and buffer spring just lube the spring with a good lube like Breakfree, G-96 or other. That will reduce any noise with them significantly.