I've got a S&W 629-3. When I rock it up and down (barrel up, barrel down), I can hear something sliding inside. Is there something in there that is supposed to float freely that would rattle like that or do I have a problem?
That's perfectly normal- it's a small safety device that looks like a key of some sort. A pin on the rebound block pulls/pushes it up and down to prevent the pistol from being fired if it should be dropped just right. Howver, there is a raised portion on the rebound block and an oposite raised portion on the hammer that connect and pushes the hammer out of the forward rest position until the trigger is released. This also keeps the pistol from firing when dropped- I don't know why the key-looking thing was added.
I'm not sure why your 617 doesn't rattle if shaken hard enough. According to Numrich schematics, it should have one unless it was dropped in later models.