Just re read the OP,
When my bike destroyed itself, the first sign of anything was feeling a severe knock like Big ends but not just under load but any time the throttle was used. Another trip up the yard just to listen again and yep, I reckoned it was the ends and getting worse! Looked further and the barrel flange was broken!
HOWEVER what I discovered inside was that the liner had also slipped down and round slightly and had been hitting the connecting rod!
My advice would be, If it sounds "Endy" and "Feels Endy" whip the barrel off and have a look! It's a lot less painful than the catastrophe that I found! I am fairly sure that my barrel flange cracked, liner slipped and then flange broke big time. However a liner can easily slip on their own although they really shouldn't. You may be feeling the rod being slowly chewed away! Check and be safe! It's only a days work!
Regards,
Julian