It sound like valve float, but could be the piston. At a higher RPM if the bore has taper to it when the crank is at full stroke the piston (with bad rings) will as you say "knock" But what is going on is ring flutter. When the rings get worn out they loose ring seat against the cylinder wall and that when you start to getting that sound at a higher RPM range. Take the piston out remove the rings and put the piston back in and measure how much clearance you have between the piston and the cylinder wall.
