The rattling is probably normal, and you just never noticed it before.
But, if the problem has gone away, it was probably what L. Mike said -- a bit of dirt/unburned gunpowder that got into a wrong place, and now is gone.
If you feel like looking inside, to blow out any loose dirt with compressed air, to lube, and just to get familiar with the workings, the keys to disassembling a Smith revolver are:
- Use screwdrivers that fit the screws precisely. Buy yourself a set of Brownells screwdrivers (interchangeable bits) for Christmas (or Ramadan, or Kwanzaa). Don't use any old screwdriver. The screws are soft, and the slots will get deformed, and will look like pelosi, if you don't use the right screwdriver.
- You don't pry the sideplate off; you rap on the grip frame with something not made of metal (a plastic mallet, a wooden hammer handle), and the vibration shakes the sideplate off. There are probably many youtube vids on how to disassemble a Smith revolver.
I took it to Brian10x, and he opened it up, and cleaned it up. Nothing out of the ordinary, and I was shocked at how little wear there was inside! The bluing on the trigger and hammer pivot pins doesn't have any wear at all! The top strap is the same, too.
We did a very thorough inspection, and cleaning, but we couldn't spot anything that would cause the lockup problem.
We think the rattling is caused by the hammer block. That's the only thing that is "loose" inside. There's nothing else that I can see that would do that.