I would also say piston slap, If you have bearing problems, at idle, you would have 0 oil pressure, that would go up a little as you revved it, and the knocking would be proggressivley getting louder and louder the longer you kept driving it like that, and, it wouldn;t go away when it warmed up. Years ago, I has a 350 that would do that. Stock, who knows how many miles, I bought it as a runner, put a cam in it, intake, carb etc, and BEAT the snot out of it. For 3 years it did the same thing. Every morning, knock for about 2 minutes, and go away.
Frank