The rattle comes from the very front part of the slide where it is unsupported by the slide rails. Since the dust cover extends to the end of the slide and fits very closely, the little bit of play allows it to tap the dust cover if you shake it just right and with enough vigor.
All of them will do it, if one doesn't it's too tight and I wouldn't trust the reliability. You don't want an auto with a super tight fit between the slide and frame - at least not in a carry gun.
The rear end of the gun where the slide rides over the frame rails is much more snug and doesn't rattle when shaken (at least on mine this is the case).
If it's reliable and shoots to point of aim, I wouldn't worry about it. Every auto in my collection has some rattle if shaken properly and that's ranging from Ed Browns to Glocks.