It's a moving part. Most pump guns rattle, or eventually do.
I've owned 870s, a 500 and a Nova SP. All rattled a bit in various places.
I've shot and carried other 870s, 590s, etc.
I prefer the 870 over all of them, but i'll use any of them cheerfully whether they rattle or not.
"And Shepherds we shall be. For thee, my Lord for thee. Power hath descended forth from thy hand, that our feet may swiftly carry out thy command. So we shall flow a river forth to thee and teeming with souls shall it ever be. In nomine Patris, et fili, et Spiritus Sancti. Amen."