The house I'm renting seems pretty old, In a couple of the walls where I've put nails in, while hammering they made this sound like something metal, small, is falling down from behind the wall... really weird, just wondering is anyone knows what this is?

You note that the house is "old". Most likely has plaster walls that are kept in place by mortar keys that overlap the lath. When you hammer a nail in, you are breaking the keys off and they fall down the wall. Not a good idea to hammer too much into plaster since it cracks and can separate fron the wall. Best idea is to drill a small pilot hole so the nail can be pushed in or just lightly tapped.