I have noticed a good rattle in my mossy as well, I don't care much it still goes boom every single time.
From handling a few diff mossberg 500's I can tell you that some rattle more than others and here is why...........
The cheaper mossberg 500, sold at big 5 and other various sporting goods stores are build cheaper. The action tube on these is that of a maverick 88, the action bars are built into the slide, if you look at the fore end of the slide there is no retaining nut. This causes it to have a little more slop, but it still works well.
The higher end mossbergs have actual stand alone action slide tubes, and then a slide that mounts to it via retaining nut. These are slightly tighter and the action bars are part of the tube (one piece) so have less movement.
The Dicks sporting goods kid prob assumes all Mossbergs rattle because the company he works for only buys and re sells the cheaper versions of the mossberg. Not saying these are bad guns i own one and I love it.
The good part is if you get a cheaper version of the gun you can replace the slide action tube if you really want to.