Just an update on this. I talked to a guy at my LGS about this noise and he said he has some ARs that do it and some that don't. No explanation though. Like I said earlier my Bushmaster does not make this noise. So I went home and field stripped both of them and noticed that the Bushmaster spring is slightly different from the DPMS. The end of the Bushmaster spring tapers a bit (on both ends) so that it fits very snugly over the buffer (it actually grips the buffer and does not allow the spring to "spin" freely if you twist it around), whereas the spring in the DPMS fits flush, but loosely over the buffer. I placed the Bushmaster spring and buffer in the DPMS and it "fixes" the noise. Anyway, I'm ordering a few Bushmaster springs and just thought I'd share in case that noise was annoying anyone else.