I'm no mechanic or frequent poster, but I'll offer my voice to the previously mentioned piston slap. Typically cause by rapid acceleration in a cold engine. Goes away because as the engine warms up the pistons expand. As the piston travels up and down in a cold state rings aren't as tight and the piston tips slightly up the bore. I'm sure a sloppy bore to piston fit could cause the same thing but you mention it was a crate engine? It's not the news I'd want to hear either but my money is betting it's not the round at the front no matter the name attached.