What could be the reason for the valves knocking on the initial starting of a 1995 Nissan Pathfinder?

Answer 1

Hydraulic lifters will rattle a little when the engine first starts if the oil has leaked out. Typically that doesn't take too long to go away. Often the knocking noise when an engine first starts is because of worn rod bearings or "piston slap". If there is too much gap in the rod bearings you'll have some knocking when you first start that will last until oil has filled the oil lines. Piston slap takes a little longer to go away, and that's because it's a noise that's caused when the skirt of the piston pops from one Sid of the cylinder to the other. A worn piston will leave too much clearance at the bottom of the cylinder, allowing the piston to slap back and forth when it's running. As the engine warms, the piston expands and usually the noise either goes away or diminishes considerably.

It's often a guessing game to figure out which part is worn and causing the problem.