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A rattling heat shield, bracket or exhaust component making a mechanical noise that the knock sensor hears and interprets as detonation. The knock sensor is a microphone tuned to a frequency band, not a detonation detector — it reports vibration in that band and the module assumes the cause, which is why a genuinely healthy engine can be pulled back on timing by a loose piece of tin.

The signature is a knock complaint that does not follow load. Real detonation appears under high load, low speed and heat, and disappears on light throttle. A loose shield rattles on engine harmonics — often at a specific speed, frequently worse at idle or on overrun — and the module's knock retard follows the noise rather than the combustion. A code on several cylinders at once favours this, since a mechanical rattle is heard by whichever sensor is nearest and not by any one combustion event.

Find it by hand before touching anything electrical. Tap along the exhaust shields, the manifold and the accessory brackets with the engine cold and listen for the buzz; a shield that has lost a spot weld or a bracket that has cracked is usually obvious. This is also worth checking before replacing a knock sensor, which is otherwise the natural conclusion and the wrong one.