If you feel that it's always the same lifter or at least the same side that
make the noise....make that test:
In the morning, engine cold, remove valve cover gasket, check gap (if any),
put an oil pan under head, start engine, listen for noise, if noise, get
your filler gauge and insert this one (.006 is good) in the noisy lifter
until you get the right thickness, when you have the right thickness, the
noise will stop and you will have what's missing. Have no worry, just be
careful.

This will tell you which one is making the noise and how big is the gap.
Try that for a few morning, if always the same lifter, same length of time,
you could have une lifter slowly going away, or it's just a bit loose... 1/2
turn in should solve the problem.

IMHO
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