Hi Ken,
Of course it could be a 'mechanical' problem, but also worth checking your timing. If it's way out (not set correctly, or slipped) it could make your motor run very hot, maybe cause 'pinking'?.
All a bit of a guess as I've not heard the noise.
It's worth finding out your piston-bore clearence. The XT piston is very short (slipper piston) so needs quite a tight fit to run quietly when hot.