I had a bit of a top end rattle on mine 18 months ago.
Had no idea what it was and only happened for a few days, seemed to quiet down when warm.
It stopped completely when the spring in my camchain tensioner snapped and bent 9 inlet valves
Now, dont panic! It would appear that this has never happened to anyone else with a Gen 1 Fazer Thou so I might be wide of the mark but..........
check it just in case.
Wish I had
And that explains my avatar of a Fazer Thou without an engine.....