How to adjust exhaust tappet/valve lifters?

[NewKleer]

Hi

my rb25det silvia seems to have a slight tapping noise in the engine, that my brother seems to think may be the exhaust cam tappet/hydraulic valve lifter in cyl 6. How do we adjust the clearance on this?

[BOOSTD]

you can't. May need new oil or new hydraulic tappets.

[blind_elk]

Or some Nulon hydraulic lifter cleaner.

[NewKleer]

guess itd be a pretty big job, especially with limited room in the silvia (thermo fan is missing the pulley by about 5-6mm). not sure exactly what is making the noise? the tappets scaping or something as they move?