I've had a rattle for the past few days and wondered what it was! It was only on start up. Sometimes it sounded like it was the exhaust and other days like it was coming from under the bonnet..
Anyway, today I had the bonnet open, and realised my engine cover was missing!!! WTF! where did it go? Its only the small one on 1.2's.. did someone steal it? but how would they I thought. It was only when I looked down by the fan I noticed it sitting there. It had fallen off and that was what was causing the rattle.. Tested by starting the car and the rattle that was there 5mins previously had gone! Its prob not the same in your case, but just thought I'd tell you anyway! Im just happy it didnt get caught in the cam belt!
Hope you get your rattle sorted out soon.. Clios, or even Renaults in general are full of them! lol...