Is it a double knock? If so it may be a wrist pin. They can sometimes knock at idle then go away under light acceleration. Try pulling plug wires at idle to see if it goes away. Removing the spark at each cylinder removes the load from the piston and can make the knock go away temporarily.
Also, are you running forged pistons? They can knock a bit when cold until the engine comes up to operating temperature.
One last thing: The cam thrust on the front of the block. If there is a bit too much play in it, it can also give a knock too. They seem to be more pronounced when cold as well.
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