Identification
The transfer bar is between the firing pin and the hammer. It acts as a principal internal safety to prevent misfire when the trigger is not being manipulated. In order to be an active safety, the transfer bar must be in the "up" position.
Testing
In order to test the transfer bar for the rattling noise, first make sure that the weapon is unloaded. Then cock the hammer back and stabilize the transfer bar with a thin object such as a pen or pencil. If you can then shake the weapon without hearing a rattling sound, you've most likely found source of the noise.
Considerations
Ruger states that the owner of the LCR should not be concerned by the rattle. In fact, the manufacturer states that the rattle is an indication that the transfer bar has the proper amount of "play" it needs in order to function properly. If it still worries you, you may consult a second-opinion gunsmith or expert to ensure the firearm's integrity or contact Ruger via their customer service department (See
Resources).