Sounded like a lifter. How is your oil pressure at idle fully warmed up?
1. Dirty thinned out oil.
2. Sticking lifter.
3. Worn cam bearings causing low oil pressure.
4. Worn oil pump
If your oil pressure is ok (at least 25 psi idle fully warmed up engine)
Maybe you just have a sticky lifter. You can try adding a quart of ATF and run it easy for a couple hundred miles, then change the oil and filter. ATF has detergents that help free lifters and helps clean sludge out of the motor.
Some people try Marvel Mystery oil, some use Neutra, some use Gunk engine flush, ect, ect. These don't always work but might be worth a try. If none of these work, might be time to freshen the motor.
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