Quote:
Originally Posted by AZKen
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?
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 
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Here's what I've done...New distributor cap, coil, plug wires, plugs, oil change, oil filter, and engine restorer, also a fuel filter. Thanks for the response. I'll pull the new plugs tomorrow and see if things clear up. I did have the autolights crossed over to NGK's and cross references have been known to be a couple heat ranges off. Timing has not been touched. Manifolds have not been touched. Haven't check the heat flapper for noise, the sounds seems to be loudest at the rear of the engine. I think if the manifolds were leaking it would have knocked when I first got it unless somehow it blew the manifold gasket while idling. lol I'll do some inspecting tomorrow. Thanks.