Engine noises are difficult to pin down when heard in person. A written discussion about possibilities can produce multiple rabbit trails. If you think it is a fuel pump, by pass the pump and remove it from the engine, you can run the engine with a cover or open temporarily (will leak a small amount of oil) but it will prove or disprove the fuel pump theory.
My vote is on piston slap. (Cylinder bore larger than piston or skirts of piston worn down)
Truthfully, talking about it will not fix it. I've heard many engine noises over the years of turning wrenches for a living and I can identify most by ear but there is always a new one somewhere that just baffles even the trained ear. Sometimes the only way is to take the engine down and explore.
About 20 years ago on my 289, I had a "misery knock", louder on warmer days as the engine warmed, even louder at times the more I gave it gas too. I had never heard this knock before in all of my years of professional wrenching and engine building. It was a "new" kind to me. When I took the engine apart exploring, I didn't really find anything at all. Even the cylinder walls still had the factory honing marks very visible. The camshaft bearings looked bad though so I assumed that was the mystery noise but it didn't look like what I heard. I had new cam bearings put in and the cylinder bores mic'd out, the bores were within specs. I began pulling the old rings from the pistons (while waiting on the rest of the block work) and one of the ring lands fell off with the removal of the rings. (The grove between the rings) I was shocked! I only needed a piston! More shocking was it didn't hurt the cylinder wall!
My example was unusual, but it is REAL. The cause: Low octane detonation. Hind site now 20/20, I did notice the destination pings in the center of all the pistons. Running Regular pump gas during the 1990's the cause. I guess the "Premium Fuel" decal isn't just a sticker!