There isn't an adjustment for the hydraulic valve lash adjusters. They are small oil-pressurized widgets that eliminate the clearance between the cam and the valve actuator. It's like an expanding cylinder with a check-valve. It expands based on oil pressure, and has a check-valve type of deal to keep it from collapsing when the cam pushes on it. Sometimes the check-valve or oil passage can get clogged, so the adjuster (or "HLA") isn't able to get a pressurized feed of oil to expand, so the clearance between the valve and the actuator increases to some non-zero value, hence the tapping when the valve is actuated (and there is a somewhat reduced amount of valve lift and duration). The only remedies are to either disassemble the valvetrain and clean/replace the HLA, or to run some sort of oil treatment that may or may not remove the clogged passage.
Some people advise using a product called "Seafoam" in your oil. I have no experience with that or Isuzu HLAs, so I can't offer any personal experience here.
VALVE TRAIN TICKING SOUND ON 3.2L ISUZU ENGINES
Valve Train Ticking Sound On
1992-98 Isuzu 3.2L Engines
Read this article regarding a valve train ticking sound on 1992-98 Isuzu 3.2L engines.
One or more hydraulic lash adjusters do not fully extend due to varnish build up inside the adjusters. This condition results from exceeding the oil and filter change intervals on the vehicle.
There are a couple of ways to possibly cure this problem that Isuzu offers. One of the simplest ways is changing the engine oil and filter using a 0W-30 synthetic engine oil prior to replacing any engine components. If the noise persists after those changes, refer to the following information.
Once we have determined which bank the noise is coming from, the cylinder head valve cover needs be removed so that we can inspect each rocker arm for clearance at the valve tip. The noisy rocker arms are the ones with clearance when the rocker is on base circle of the cam and the valve is closed as shown in Figure 1.
Note: Each cylinder head has an oil pressure relief valve that controls oil pressure to the rocker arms. If all rocker arms have clearance and/or the rocker arm pivot shaft is worn, then the cylinder head oil pressure relief valve may be stuck in the open position.
Remove the rocker arm(s) that have clearance on them and inspect the wear pattern of the hydraulic lash adjuster surface contact with the valve stem as shown in Figure 2. Then remove the hydraulic lash adjuster from the rocker arms using your fingers. Do not damage the O-ring on the outside of the lash adjuster or replacement of the entire rocker arm is required.
Insert a paper clip into the hole at the top of the lash adjuster and depress the spring loaded check ball while completely pushing in the piston at the opposite end. Some traces of oil may come out of the check ball hole.
Carefully remove the O-ring and spray the lash adjuster piston with cleaner to remove any varnish while holding the piston open. Submerge in cleaning solvent with the check ball depressed and pump the piston repeatedly to allow the solvent to penetrate through the lash adjuster.
Submerge the adjuster in clean 0W-30 synthetic engine oil with the check ball depressed and pump the piston repeatedly to allow the engine oil to penetrate through the lash adjuster. Then allow the piston to extend fully to fill the adjuster.
Reinstall the O-ring and lube the rocker arm bore that houses the adjuster with 0W-30 synthetic engine oil, then push the adjuster back into place. Inspect for leaks at the check ball and piston by attempting to compress the adjuster without depressing the check ball.
Then you can do it all again in a year or 2 when it happens again. Hydraulic lash adjusters tap, theres really nothing you can do about it. You can get them quiet for a while but it'll come back.
The mistubishi boards are full of posts about the same thing. Mitsu came out with replacements with a larger hole in them which helped but didnt stop it and after a while they sounded just like the old style. Just get used to it.