Help with engine rattle after warmup...

First, thank you for letting me join this message board. I recently purchased a 1990 Mazda B2200. It's the carbureted, 5 speed model. I have an issue that I can't seem to pin down an answer for. The truck has a ticking near the back of the head only after the truck has warmed up and stays there until I let the truck sit overnight to cool.

Now, a little background on what I've done so far: The truck was in need of a tune up, so I replaced the plugs, wires, cap and rotor. I also changed the oil to 10w-30 and replaced the fuel filter. I added some Marvel Mystery Oil to the engine oil to clean any sludge from the engine. Thinking that what I could have was the dreaded HLA noise, I checked the lifters. They seem to be fine and holding pressure.

I probed around the engine with a stethoscope to try to locate where the sound is coming from. It appears to come from the mechanical fuel pump. I replaced the pump to no avail. Is there anything that I may be missing. The truck runs fine, as is, but I can't seem to find what is causing this noise.

It's likely the HLA's and the sound was transmitted to the fuel pump vial the arm on the fuel pump. It isn't likely you can really test the lifters but it is possible to clean them. A set of replacement units is also a good option because they don't cost much.

Thanks for the input. The only thing that confuses me is no rattle when cold. Loud rattle when warm. And the noise is coming from the pump. It slowly fades in as the engine gets warm.

My 88 was like that, never had a cold rattle from the lifters, but it was damn consistent once it warmed up. As the engine warms up, the oil gets thinner and the oil pressure drops. The lifters in these trucks make noise like clockwork, some when warm, some when cold.

The lifters in my original engine in my 93 were nice and quiet, until you changed the engine oil, then they would tick like the motor was going to blow for about 20 minutes when started cold, then go quiet after 20 minutes, it would do this for about a week after changing the oil, then they were always quiet cold or hot, was the weirdest thing, but they always did this, even if my first trip after an oil change was a couple hundred miles or 2 miles, it was 1 week before they would shut up.

If your fuel pump pushrod was making the noise, it should do that all of the time, because there is no adjustment to the freeplay, but I seem to thing that the fuel pump keeps pressure on the pushrod for it, so it may be worthwhile pulling your pump to see if it is all good, there should be 1 plastic spacer between the pump and head.

I found my issue. I have a notch worn into the fuel pump lobe. Every time the pump arm hits the notch, it jumps.

You must have been missing the fuel pump spacer at one time... or even still missing it.

When I changed the pump, even the old one had a spacer. But I do know that the head has been reworked before. Whoever done the head etched the rockers with their respective position. It is possible that they forgot to put the spacer on at first and realized their mistake after they ran the truck. Needless to say, my next approach is to convert to an electric pump. Any advice, as the mechanical pump has three lines instead of two. What should I do about the return line?

You won't need the return line, the pump will stabilize at pressure. You can plug it. Put a fuel filter between the tank and the pump.
I would change the oil pressure switch to one with a normally open extra circuit in it and run the pump through it. Trust me you do not want a fuel leak/fire to keep pumping on the fire. Don't ask how I know you do not want that. But the engine dies, oil pressure drops, pump stops.