that looks like an m40, 1.6??
valve lifters are hydraulic, and not adjustable. if the mileage looked correct, you have 274k kilometrs. these things do not last forever.
you could try some lucas oil treatment to see if that quiets them, but you may very well be looking at replacing them.
i would do the entire set, though, if the oil treatment doesn't help.
cd2 is also a good product, but not sure if available in your neck of the woods.