OK, let's consider ALL the possibilities [which the posters below have touched upon]. Either [A] the knock is directly related to the changing of the plugs; or [B] the knock is NOT directly related to the changing of the plugs.
So first, pull the plugs out one at a time, and compare them with the old ones. Make sure they have an identical thread reach. If that's OK, then look at the electrodes. If the side electrode looks like it's been hammered flat to the center electrode, SOMETHING is making contact with the plug.
If it's carbon, an old time mechanic's trick [technicians don't use tricks; they use fancy analyzers and scanners and such] was to start the engine [replacing the plugs, of course], bring it to a fast idle, fill a 6-1/2 ounce Coke bottle with plain old tap water, and SLOWLY trickle the water into the throttle body [or carb, back in the old days],continuing until the contents of the bottle are gone. When the fuel in the cylinder ignites, the water in the cylinder vaporizes from the heat, and it effectively "steam-cleans" the carbon out of the cylinder. The water also helps break up any chunks of carbon that may be in the cylinders, too. [Ever notice that when you have a blown head gasket, the leaky cylinder is the cleanest and has the least amount of carbon? Same principle.] Or you can buy a can of Hastings Products' "CASITE" and do the same thing, but the neighbors will be convinced you're fogging for skeeters before you're done. But with CASITE, as I recall, you pour in about half a can, and then start pouring faster until the engine stalls...then let it "soak" 30 minutes before re-starting the engine. At least that's how I remember doing it in years past.
If there's NOT an [A} scenario going on in your Duck-odor, then obviously it's simply something coincidental that "chose" that moment to make itself apparent. Good luck tracking that down, because the only way to do that is to be there with it...which I'm not. But the other guys have given you some good ideas on which directions to look.
Again, good luck with it, Lanse. I believe you have the makings of a good mechanic.
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