20w is awful thin viscosity. Old oil ends up that thing after a few thousand miles. Try putting some 50w oil in along with some STP or same type additive. It sounds like your engine is on its death bed with the rattling. Usually that means the connecting rod bearings are worn out. That usually happens with age, but also with low oil. After a while the oil seals leak and it only takes a few seconds of no oil to the bearing surface and the bearings and crank need rebuilding.
Let someone who might be familiar with engines listen to it. They don't have to be a Toyota expert because all bearing noises sound similar. There is no fix other than pulling the engine and getting it rebuilt or just replacing the engine.
Let us know.
Russ
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