With time, valve lifters can get clogged up or sticky with extensive use. Hence, why it's crucial to have them cleaned and restored, especially if you're keen to solve issues like lifter tick. Although most experts recommend removing them before cleaning, you could still easily clean the valve lifters without having to remove them from the engine. Often, this would involve adding an engine flush cleaning solution into the oil reservoir and running the engine for a bit to let it circulate. Once it's had time to work at cleaning your engine (including the lifters), drain out the solution (with the oil), and perform an oil change. This includes swapping out the oil filters. Then, add some new oil, and that alone should help to clean the valve lifters without having to remove them.