Then don't shake it - just kidding. A rattle most likely means that one of the internal optical components came loose inside. This is usually caused by dropping the instrument. Please send the instrument to our repair department. There is nothing you can do in the field to repair it. In the future remember that this is a precision optical instrument.
Although we try to make refractometers as rugged as we can, please try to handle them as you would a pair of binoculars or a camera.