Hi,
I have a non dig-s model but I think the bottom ends are very similar/the same, does the rattly noise only happen at idle? And does the whole car vibrate very gently at this point? If so I believe it is because these engines have 3 cylinders, causing them to be imbalanced, and the balancing applied is tuned for when the car is driven and not when it is idling, causing the engine to shake quite a lot at idle with no load (lights off, A/C off etc.).
Mine idles at around 750rpm with everything off, so I think this is normal too. Once the A/C is turned on the engine revs higher, due to the A/C compressor using engine power, to around the 1000rpm mark on the dial. I think modern engines idle so low to reduce fuel use at traffic lights etc. for models without stop/start.
Hope this helps 