If I rock the gun forward and backwards in the upright position, I get a clicking noise.  Almost like something is sliding inside from end to end making a scraping and then clicking noise as it gets to the end of it's slide (if that makes sense).  Is this typical?  Do you have the same thing with your rifles?  I'm hoping I didn't mess something up assembling the lower.  I know it is coming from the lower as the noise also happens with the upper removed. Everything appears to be working correctly, although it has never been fired yet.  Thanks in advance!!

If the extension is "populated", likely the buffer and its spring (normal). Remove them and retry.

Buffer weights?

This; what he said. Weights.

Pull out your buffer and try it, now move your buffer. It is by design.

So, this is normal, and nothing to worry about, correct?

The eliminate the buffer weight shift noise swap what you have with a Spikes Tactical/Kaw Valley Precision T2 Tungsten Buffer. Both use powered tungsten in their buffers so no weight shift.