The car was parked for about 15 minutes when I came back. I started it up and turned on the rear defroster. There was a rattling noise coming from the rear of the car. Normal?

Definitely not normal. The rear defroster is an electric circuit. The lines on the rear window are a conductive silver. The window is heated by the heating of these lines as current runs through them. The electrical resistance of the lines causes the heating of the glass. There is nothing in the circuit which should rattle. I would look for another reason than the rear window defroster.