Very strange.... need to narrow down the area where the rattling is coming from. Can you tell if its more apparent on the top of the engine, coming from the valve cover area or if its more from under the car around the oil pan/ crankshaft area?
Intermediate levers, timing chain guides or noisey valves are typically from the top of the engine and crankshaft, piston rod bearings and oil pump make noises from the bottom of the engine.

If you're able... try putting in a thicker oil (10w-40 or similar) and see if it eliminates or reduces the rattle?

Hope others have better information...sorry?

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