When only one hydraulic lifter fails in the engine it won't allow one of the valves to open and close like it's supposed too so you might notice a misfire in the engine.
Before hydraulic lifters fail though they will usually make a clicking, tapping noise that seems to get louder the faster you accelerate.
Most times noisy lifters are just an annoyance and don't cause much if any damage to the engine.
The noisy lifters will usually make a clicking, tapping noise etc and gets worse as you accelerate.
You can sometimes add an additive to the engine oil to help stop the lifter noise and sometimes it requires replacing the noisy lifters.
Fixing lifter noise in an engine usually requires disassembly of the intake manifold or other parts of the engine to get to the lifters.
Sometimes the lifter noise will go away by using some oil additives such as Lucas and then allow the engine to idle for awhile.
But if the lifters are really damaged and failed then they will need replaced.
If you do it yourself and replace the hydraulic engine lifters you could likely do so for $150.00 or less and the time it takes to replace the lifters.
If you have a shop do it it could take the shop 4 hours or more and the cost is around $500.00 to $800.00
I've driven an old car another 50,000 miles before with hydraulic lifter noise and it wasn't worth repairing.
I drove that car to work and back and paid $500.00 for the car and it had lifter noise when I bought it.