I've just sorted out mystery "spark knock" in my '97, 4300 Vortec. My situation is similar to yours, brand new extreme quality tune up with hi-po parts - no good reason for this to occur. In my studies I found that if I filled my tank with super unleaded (91 octane) the problem would solve itself, only to return if running 87 octane.
Hmm. Obviously problem not solved - it's not tuned good enough. Grr.
Owning a Vortec has proved a rather steep learning curve, working around all the computer junk I so loathe in order to make the truck do what I want it to do. Look, I didn't buy it nor would I have, TOO NEW.
Beside the point. Turns out the stock distributor has right around + or - THREE DEGREES worth of play in its' lock down. Obviously, if the truck knocks under load, how the heck am I to adjust the timing? Stupid Vortec, thinks it's so smart. I'll show it! Given years of mileage and trucking, I assume the distributor loses time IN THAT "adjustment" and this is what I found to be my solution. It does NOT seems like much, but it worked for me. Maybe it will work for you. Clockwise to advance, counter to retard.
It reminds me of a similar solution found when working on FORDS. The SPOUT connector is worth three degrees retard and that often solves the owners' knock. Loosen the distributor lock down tab and wiggle it back a hair. This has been a proven solution upon successful conclusion of the 91 octane test. If your truck purrs like a kitten running 91 octane, I would jump at my offering of a fix, meager as it may seem.
If it was a physical issue like carbon build up on the valves or pistons -something similar, it would do it ALL THE TIME. I suggest the 91 octane test as mentioned above.
