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Has this truck been sitting for an extended amount of time? Ive encountered two instances of were this has happened because a vehicle sat for an extended length of time and a mouse chewed through the wires under the intake manifold. Check for evidence of mice in and around intake manifold.
Have you inspected in and around the intake already? A bore scope is handy to check under intake without removing it. Just because it didn't sit for an extended time does not mean it couldn't be rodents.
Inspect around engine computer for any water intrusion. Remove connector off engine computer and look at pins for any sign of green corrosion.
The next step would be to remove intake manifold and preform a resistance check from the knock sensor connector pins to the ecu pins to verify you have a complete circuit.
Have you inspected ECU, and connectors? Did you end up removing Intake manifold? Do you need any more assistance at this time, if not please accept my answer at this time.