Do you have a tapping noise when cold? Try this when it is warm, Rev to 3000 rpm and hold for a slow count of 20 then let it idle for a count of 20 repeat this about 10 times and you may well have cleared the problem.
The hydraulic tappets may be dirty and sticky. An oil change may also help. If the above method does not clear it then you might need to replace the lifters as it is probably a faulty lifter valve.