Motorcycle Repair: Noise like tappets that is not tappets, massive apologies, bike mechanics

Question
Massive apologies if this is not one of the bikes you help with, but this is my Husbands bike, and I am not sure what actual make it is other than a Honda 650 Bandit.  If it is not your area, please let me know and I will re-ask someone, but I couldn't see anyone who knew anything about them on here.

Anyway, the problem he is having is that there is a rattle from the engine that sounds like a tappet, not when Idle just when throttling up? (is that the right term? can tell I'm a car driver eh!) He has had it in the repairers for nearly 2 weeks now and they have done the tappets, checked/changed the slippers? changed the belt, and are now looking further into the engine, but STILL cannot find it.

Do you have any idea what it might be?  The mechanic has been great and they have been established for over 30 years as bike mechanics, but are stumped.

If you need more (precise) info, let me know.
Many thanks,
Denise

Answer
I am not familiar with this bike, but you may be hearing 'pinging' which is an indication that the ignition timing may be too far advanced, or the fuel grade may be too low.  Since bikes normally run on low-grade fuel, unless the compression has been increased by tuning, the problem probably lies with the ignition timing.

Good luck,  Jan