I'm amazed at how many people try to tune without using something to watch what the ecm is seeing. It takes years of experience to even try to be good at it. You have spent more time posting about knock than it would likely take to identify the problem if you could get something to watch what's going on from the ecm's point of view.
CEL can illuminate if knock is detected and cannot be eliminated. The code assumes the sensor has a problem and is falsely indicating knock.
"piston rattle from knock"... <sigh>
Well, the guy's probably a damn good engine builder, but "piston rattle" is a condition caused by excessive clearance between piston and cylinder wall, and "knock" is a condition caused by fuel combustion causing excess pressure which cannot be converted into mechanical energy by the piston and crank and is instead converted to heat and sound energy. The have distinctly different sounds. Piston rattle can occur with the spark plugs disconnected, as it has nothing to do with fuel burning and has everything to do with the "fit" of the piston. Knock will rarely occur with the spark plug disconnected as it's caused by fuel and air reacting, which is usually started by a spark.
You don't need to remove the cylinder head to eliminate carbon buildup as a possibility. An old quickie trick is to use water to remove carbon. Get the engine hot, bring rpm to about 2500, and slowly pour a cup of water into the intake without stalling the engine. That will generally remove some of the carbon. It's tricky with a PFI engine but you might be able to use a piece of turbing on the TB to get the right angle to make this work.
For an absolute, guaranteed, sure-fire way to remove carbon buildup use GM Top Engine Cleaner (DO NOT use Sea Foam instead. It is no-way, no-how not the same, and I'll be glad to tell why if needed). Get a can of the spray version of TEC, and some tubing from an air shock "replacement hose kit." Get the engine warm and spray some TEC (about 1/2 can) in the TB while the engine is running. Immediately shut engine off, pull plugs as soon as you can, and using the long tubing spray an equal amount of the remaining 1/2 can in each cylinder. Replace plugs, let engine sit overnight, and start in the morning. Don't start it inside your garage! It will smoke very badly. Run the car until the smoke clears up then go for a drive. If there was any deposit buildup large enough to cause knocking, it will be gone when you're done.
hth
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