I have a 95 Nissan pickup, 2wd, 4 cyl, short bed, 105,000 miles. I bought the truck when it had 14,000 miles on it, and it has always produced a slight knocking noise when cold at ~1500 rpm or so, no load. No load means I push the throttle down just enough so that engine is neither accelerating nor deccelerating the truck. This goes away when warm. I had the dealer look at it, they found nothing wrong. I have learned to ignore it.
Lately, the truck has developed a "knocking" noise when cold at ~2500 rpm and up, under load only. It's a moderately light knocking, sounds a bit like piston slap but I can't be certain. It's too deep and heavy to be an exhaust sheild rattle, if you have heard pistion slap before, it sounds very much like that. I don't think it's a loose wrist pin because that is not usually load or temperature dependant. It could be a lifter, but would a lifter noise be load dependant? It completely goes away after about 5-10 minutes of operation.
So, my questions are, is this something to worry about, is it normal for this engine to do this at high mileage, and is it a sign that the death of the engine is approaching? I might mention that the engine does not burn any oil. I use 5-30 oil, but I had one oil change where they put "high mileage" oil in the engine. I'm not sure what that is, I have visions of them dumping a container of stp into the engine.