Knocking 340

If it is piston-slap,
and it really does go away after warm-up,
then it is probably nothing to worry about.
Some factory-type 340 pistons experience a lot of growth as the heat gets into them, and so they have to be installed looser than what we might consider normal.
Piston slap is a different sound than a rod knock, which is different from loose piston pins , or even a fuel-pump arm slapping on the eccentric. Get an audio clip up, and someone will help you. Or get an old timer to listen to it.
If the engine is not in the car, you have to work at it as a process of elimination. Eliminate the pump, then the wrist-pins, then the rods, and then you are left with only loose pistons. If the heads are off, it's easier. If the pan is off, even easier.
Loose-fitting pistons, of themselves, at start up are not a big deal.

But; having said that; KB 243s are hypers and can be installed very tight, even for HD use. So I would be surprised if they were slapping. So then it is likely in your best interest to at least take the pan off and go hunting. I agree with all those that have mentioned it before me.

Also; with the engine in the car and running, it can make knocking sounds originating NOT inside the engine. Some other sources are; the flex-plate, headers, manifolds, transmission rear mount.

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