Engine knocking

I ride a 525 supermoto and the engine knock pretty bad now. I was told its the piston so i changed that, gave the conrod abit of a wiggle at the same time to see if it could be the bearing but it seemed fine.

Was then told by someone else its the clutch cause when you pull it in it seems to go quieter, he said to check the nut on the end of the crank because it has proberbly come lose, but it hasnt and there was no real end float in the clutch.

I read on another thread how that if you change down gears to fast the back wheel spins faster than the engine and f*cks up the gearbox unless you have a slipper clutch, is this true? I know the wheel spins faster thats why you drop it down 2 on the corners to get the back end out.