could be that one of the lifters is not getting enough oil. I have a similar problem with my bmw. I use fully syntetic penrite 0w 40 and since i started using this oil the lifters are much quieter. I also did the oil flush before i started using this oil. one or 2 will start making noise if i drive it gently thru the city, but a nice 5 min spirited drive fixes it. I guess it needs revs to get the oil to the failing lifters.

Im not saying that you should use this specific oil in your car, just giving an example. My engine is 20 years old so this oil is much better that the oils they were making 20 years ago.