I have had this problem before, and one of my previous jobs was an engineer for lifters and lash adjusters. They are minuature hydraulic cylinders with VERY small clearances and a check valve. They way they work is that the check valve opens when the valve is closed to fill with oil, then as the lifter needs to push the valve open, the camshaft pushed on the body and the pushrod pushes against the plunger. The oil is more incompressible than thw spring, so the valve opens. However, since thete is a slight clearance between the body nd plunger the plunger will move over time. That is how they adapt to valve wear, so ypu no longer have to set lash.
When a piece of dirt gets in the high pressure chamber, it can block oil from entering and cause the lifter to collapse (that is the tick).
Typically, when you have that ticking sound, 1st check your oil. That is the first sign that it might be low.
If the oil level is ok, then try a detergent. I've used diesel fuel, MMO or ATF should work as well. Drain a quart of oil from the pan, pour in the detergent, and run the engine for 10 to 20 minutes. Give it a fresh oil change and see if that fixes it. The detergent is trying to dislodge or clean the minute passages within the lifter, so that it can work properly again.
If that doesn't work, then they are worn and will need replacement.