If you swear on a GMC manual that you did nothing else !, then play with timing. If no cure, recheck that the plugs are the same and put old plugs back in to see a difference in sound. Obviously it is something you did and not an all of a sudden, coincidence. So backtrack and think. Hotter plug? Colder plug? wrong length plugs? moved timing? .... loosened manifold or crossover pipe?, high octane gas. Watch the temp gauge so as not to damage motor during trouble shooting. Need to replace items, tighten manifold bolts and re-time to get any kind of a change, this will lead to the remedy. Do one thing at a time and test or you will get confused/misled.
Be aware that a leaky exhaust manifold can make big knock sound. Did you check the heat riser flapper for noise?
Bottom line: Get a broom stick and hold it all around motor while up to your ear to isolate the noise if you can. Rich guys use stethoscopes
