Chevrolet - 1500 :: 2001 - Long Lasting Engine Knock

I have a 2001 Chevy 2wheel drive 4.3 V6 Pickup with a long lasting engine knock. I use it only to haul trash and building materials, maybe once every other week. The knock is sporadic. If I take a long trip and drive on an interstate highway the following morning upon start-up the truck will knock heavily and continue to do so as long as it is running. If I take the truck for a short local trip, the following morning it will start with no knock and only start knocking after driving maybe 10 miles. If I drive only short trips, the truck will almost always start without knocking and only knock once it is fully warmed up. I purchased this vehicle from my brother who drove it this way ever since he purchased it at a salvage auction (full of dents and some damage) who put over 20,000 miles on it with this same engine knock.

Other than the dents, the truck is in great shape, no rust, and has 118,000 miles on it. I would like to keep this thing because it was very cheap and I only use it sporadically. So, I'd like to know if I'm going to eventually have to put a motor in it or if there is some small fix I could make to stop this knocking sound. The sound: It is heavy. It sounds bad. It knocks along with the rpm of the motor. My brother, who is also a great mechanic, thinks it may be piston slap or something in the valve system. I've also had a mechanic listen to it. He thinks it's a rod bearing or wrist pin. What I don't understand is how could it be any of those things when it makes a difference how I drive it the last time it was used? Could it be something simple like a vacuum issue, knock sensor or crank sensor?