Just went through this, after adjusting the valve gaps three times, using a stethoscope and thinking I had a piston slap.
The last thing I checked prior to an engine tear down, was to check for a exhaust leak, which sounds a lot like a knock or valves way out of adjustment.
I had 2 small cracks in exhaust header manifold, which I had welded. Upon reinstallation, motor is noticeably quieter, more like a sewing machine.
To find the header cracks, I used a shop-vac with the hose plugged into the rear of muffler and reversing the air flow into the exhaust. Next I used a water and soap mix in a hand-sprayer to spray all joints, the entire exhaust system and exhaust manifold. Shop vac puts out enough air pressure to produce large bubbles from the water/soap mix to spot the exhaust leak(s) in about 10 minutes of searching.
Good luck