Whatever you do, don't do what my father did when trying to hunt down a strange rattle in the back of a Skoda Roomster.
It was only when I noticed that there was no rattle when I was driving it alone—which I rarely did as it was poo (underpowered & automatic)—that we thought of checking it with the rest of us, and the common denominator was my father.
It seems he kept putting his walking stick in the car at a strange angle which make it look completely secure, but the handle just taps against a bit of plastic near the rear seats, causing this rattle. Of course when he's a passenger or not in the car it doesn't happen.
I think it went back to Skoda about 8 times before they told him that they'd start charging for diagnosis as they'd spent so much time on it already and had not once been able to duplicate the noise.