Engine of my bike (Honda) produces cracking/knocking sound. What could be the possible cause?

I have checked the oil level using the dip stick, it is in good level.

There's no cracking/knocking sound when the engine is idle (onand running).

I have checked the chain, the tension is ok. Not too tight, not too loose.

Before I hear the cracking, a sound similar to rubbing rubber begins until I hear a "Tack" "Tack" "Tack" that seems like a metal rod is hitting the crank case. There's no constant interval with this sound. This usually happens when gaining and reducing speed. Between 2nd and 3rd gear.

When I started to switch on the engine in side stand position, continuous drops of oil is suspected coming from sprocket shaft.

Case solved. The bearing of sprocket shaft was broken. This is due to wear set of sprocket and chain that make the shaft wiggle due to torque when accelerating.