Your Xterra has hydraulic lifters and they are not adjustable. If they are noisey then either you are using the wrong oil, the wrong filter, or your lifters are bad. If you do a search for noisey lifters you should find lots of discussions about what is normal and what is not.
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ChuckH
"Every morning when I wake up I know it's not going to get any better 'til I go back to sleep again!" Al Bundy