The engine has a timing chain, which could be the source of the rattle.
If the engine has done high miles then it could be worn.
Alternatively, keep doing oil and filter changes (more frequently than normal) to ensure fresh oil is getting to (and staying) in right places.
You could try something like Slick50 if it is still available to buy to ensure the tensioner and chain is lubricated at all times.
Good quality oil of the correct grade and specs (e.g. API spec) and an Original Equipment (OE) specification filter is used as this will make a difference