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Crankshaft or camshaft bearings worn beyond their clearance, which shows first as a loss of oil pressure rather than as a noise. Oil pressure is the result of a restriction — the bearings themselves — so wear opens the clearances and the same pump can no longer maintain the same pressure.

Hot idle is where it appears first, because that is when oil is thinnest and demand lowest, so pressure falls there while remaining acceptable at speed. A pressure warning at idle that clears when revved is the classic early presentation, and it is frequently dismissed as a sensor fault because the gauge looks fine while driving.

It is also heard as knock at the wrong frequency. A knock sensor listening for detonation will report mechanical noise from a failing bearing, so a knock code on an engine with low oil pressure is worth taking seriously as a mechanical finding. Confirm pressure with a mechanical gauge before concluding anything — and inspect the sump for bearing material, which converts a suspicion into a certainty and changes the conversation about whether the engine is worth repairing.