Well, my experience of gearbox issues are that the 'rattle' goes if you depress the clutch, and it is prominent in gears 1-3 at any speed, so it doesn't sound like that from what you say. As, above, there could be a number of reasons for the noise, could it be induction?
I found some conkers in the air box of our blue 1.2, which rattled loudly. I can only think that they were picked up into the air intake when the car was driven down a country lane or ? They were lodged under the air filter in the housing, so noises can come from some odd sources.