What could it be when you start the engine it sounds dry like you just changed the oil in the truck, but it does it whenever it has sat for more then a couple hours. It never use to do this before.
It means your main/rod bearings are so loose, that until they fill with pressurized oil, they hammer…You can expect to “throw a rod” before much longer. NOW is the time for 20-50 oil while you save up the down payment on a new truck…
A tired engine comes to mind.
A 300 CID inline six is a good engine. But any engine with 400,000 miles on it is going to show it’s mileage, and start displaying a lack of oil pressure.
“You can expect to “throw a rod” before much longer”
OTOH I have a friend with a 300K+ mile '73 Chevy van that always shows oil pressure at 20 PSIG when warm. It has hammered on start up for the entire 22 years I’ve known him. He tows a loaded car trailer hundreds of miles at a time with it. You can’t argue with success.
I’m inclined to agree that it’s just wear on the bearings catchin up with the ol’ engine. But as MG said, it could run for another 400,000 miles like this. Since the noise is disappearing it means that a pressurized oil barrier has successfully filled the spaces between the wear surfaces and the sleeve bearings on the crank and mains (and rods) and the wear surfacea are protected.
The only way to confirm the theory would be to tear open the engine’s bottom end, and IMHO that would be foolhearty. I’d just think of it as “old engine eccentricities” and live with it until the engine bacame unreliable or started smokin’. You can allways rebuild it then…if “then” ever actually comes.
No sense breaking open an engine that’s runnning well just because of a little cold-startup rattle.
“it could run for another 400,000 miles like this.”
No it couldn’t…
It could be a defective oil filter (anti-drainback valve in filter may be leaky). I would try replacing the oil filter first, with some brands of filter (Napa for me) i’ve had it start dry every time, but when I use a quality filter (purolator pureone) the oil pressure is up the instant the starter stops cranking.