Edit: my rattle goes away with the clutch down, it seems the clutch is pulling whatever is lose tight again.
Well it's a small world! I have exactly the same thing with Sir D - I've diagnosed (with assistance) down to 2 parts - Clutch release bearing, or the idler gear. Sir D's engine is being hoisted out March/April to address this and the speedo drive. If you dont get yours done before then, feel free to take note and visa versa. :)
To clarify - when cold it's barely noticable (because i know to look out for it, I can hear it), once warmed up after a 25-30 miles drive it's far more apparent and easily distinguishable for general engine bay rattle. Clutch down and it's gone.
Edited by nev_payne, 01 February 2012 - 11:39 AM.