I recently owned a '96 Divvy which I bought with 18,800 on the clock. I had the same noise with mine as soon as it got hot - sounded like a bloody woodpecker was trapped in the engine . I did an oil change but it didn’t make a lot of difference.
Anyway, I'm a mechanic in the Army so I stripped it down to check the valve clearances and they were all fine. I wasn't convinced, and as this was my first big bike (passed my DAS on 3rd Aug 05) I took it to the Yamaha dealer for a second opinion. Their mechanic had a quick listen and laughed at me - 'you've got nothing to worry about,' he said, 'that sounds sweet as a nut, they normally sound like a tractor.' Anyway, he then had a listen to the clutch release bearing and said that I had no problems there either - apparently they make quite a row sometimes too.
Since then I’ve had a listen to my brother-in-law's '98 Divvy with 24,000 on the clock and his was even worse than mine.
Moral of the story - I would say you have nothing to worry about my friend