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Some OEM codes have a background knock learning strategy. What this means is that the ECU will try to learn in the background noise created by all moving parts. How this helps is once this background noise is learnt it will allow ECU and knock sensor to pick up knock more accurately. Too low a background learn will make knock sensor more sensitive to knock and too high a noise learn will let low intensity knock pass through. When we relash ECU this learn is lost and most companies resort to a safe background noise calibration. This makes the sensor pick up knock that would otherwise be ignored and perhaps is why the more stronger spark pull out initially. Background learns are completed at low load from mid to high engine speeds so almost like coast down in gear sort of or 1st gear and slow engine speed increase at low load.
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