HiIt has 25k on the clock and I don't think the valves have ever been checked, I'm worried they are out of whack. I've spoken to a few mechanics from the book, one said they don't work on bikes that old (1992), the other said he couldn't be bothered to do it!
Enjoy your motorcycle, and ride safely.
For recommendations to good mechanic I suggest you take recommendations from motorcycle riders in your neighborhood.
Valves do not tend be come out of tune, unless you are riding the motorcycle really hard and at a high RPM all the time. It is possible that your valve clearance has never been checked and is still normal. Noises of the type you describe may be due to another common cause: Faulty timing chain tensioner.
If you trust your mechanic explain to him about the noise and let him decide what needs to be fixed, and maybe he will calm you and say it is normal noise?