is your engine dirty from inside? you might need to run a cleaner in the engine before oil change - it might rectify the lifters, if not then buy 6 new lifters and replace all of them, they come in a very hard blister pack as new parts. These are very long life items as long as you dont let the mechanic mess with them every time.
Before replacing the tappets and you are sure your engine is super dirty remove the camshaft and remove each lifter one by one and keep them in the correct order - clean the tappets with kerosene and some fresh ATF, if you are taking the lifter apart then very carefully label each one so their internal parts in the correct order - dont mix up internal parts or even swap the lifters from one valve to another.
It may correct itself.
Now lets assume you have installed new lifters and still have noise then check oil pressure and cleanliness of the engine.