Valves probably need to be readjusted back to spec. Get a service manual for your vehicle to look up the procedure for doing this. Generally just need basic tools and a feeler gauge.
are you sure is a valve noise?? or is a piston slap?
Wear or improper adjustment.
If clicking while trying to start then might be a bad starter. If clicking while running, then might be valve/lifter noise.
a broken rod or valve lifter
The PCV valve on a 1999 GMC Jimmy is located on top of the engine, on the left side. It connects the valve cover to the intake manifold.
It retards / slows the engine timing up when it detects an engine knock / pining noise like bad fuel are valve noise.
does it have a ecotek valve fitted?
Engine running hotter then 220 degrees can cause valvetrain noise. Bad fuel can also cause valve noise / PINGING sound. Ignition timing to high / Advanced to far, can also cause engine noise.
The noise after flushing the toilet is typically caused by the water refilling the tank and the valve closing.
A clicking noise in the engine of a 91 Ranger could indicate a problem with the valves. A valve lifter in the engine may be worn or stuck.
you have a stuck valve
Bearing noise is usually deeper and comes from farther down in the engine. Lifter noise is more of a ticking and is up in the engine heads.