there are 2 knock sensors on that engine, both under the intake manifold.
These are pretty common for failure due to water intrusion and/or debris contamination.
there should be a foam block under the intake, this is 'supposed' to keep debris out from under, many times it is removed when the intake gaskets are replaced ( another common failure )
The sensors sit in holes in the valley, they wil have individual codes for front or rear ( 1 or 2 ) so know which is which before you start throwing parts at it.
Intake is easy to pull, be sure to properly torque the sensors ( use no power tools !! ) and the intake bolts upon reassembly ( and use lock tight on the bolts )
