Engine knocking noise

[TheMask]

I got the bike with a significantly knocking noise on the left part of the engine, almost identical with the one in this video.
- the sound is persistent on idle.
- it remains when I pull the clutch, but with a very small intensity reduction.
- it remains even when I'm in the first speed with a low engine loading.
- completly disappear on loading when I start to pull the gas.
- if I ignore this sound the engine run smoothly and have a nice, vigorous sound.
At the end of this season I'll investigate the problem by opening the engine. My questions are: does anyone had the same problem? how he has solv it?
Thank you, guys!