Engine Knocking at High Speeds

Confirm the symptom by operating the boat at the speed where the noise occurs (above 35 mph). Distinguish between a metallic 'pinging' or 'marbles in a can' sound (detonation) versus a rhythmic, deep 'thumping' or 'hammering' sound (mechanical/drivetrain). Perform a neutral rev test: if the noise persists in neutral, it is likely internal mechanical wear; if only present under load (in gear), it is likely detonation or drivetrain-related.