If it sounds as you describe, then you really need to have a look inside. I’ve seen loose movement tabs and screws, rotor screws, bridge screws, etc, and you don’t want something like that getting into the gears or hairspring. If you don’t have a caseback tool then a ball of duct tape works for some people. Or take it to a local watchmaker. I wouldn’t recommend wearing the watch until you determine what’s causing the rattle. If it’s just the normal sound of the rotor then you’re ok. If it really sounds like shaking a can of paint, then you’re not.