1995 isuzu Rodeo engine noise

valve guides worn and tappet adjustment required, can check tappet adjustment first to see slightly better before removing heads and getting valve guides repaired/replaced on heads common problem
if raise revs to 1500-2000rpm and hold does clatter come and go come and go (valve guides) if there all the time tappet clearance issue
if there only under load (1st rev up) for a couple of times then stops possible big end bearing
all these are common for this engine

I'd advise you run a compression check on all the cylinders. I believe the 3.2 Isuzu engine for that year is still a pushrod engine. You could have a bad lifter that might be sticking. You can also use a stethoscope to listen for any odd noises. Unless you have a broken rod. I'd have a helper start the engine while your at the front with the hood open and listen for the noise, figure out where its coming before you can really determine your best point of action.

change oil and add some oil stabilizer to it