Fresh Rebuild; not starting
The noise could have been a number of things, But here are the questions that I have that I don't think was answered in the post, First off were the lifters primed prior to installing them? how long had the motor set before firing from rebuild? Was the oiling system primed before first start? Just from what was said I think lifters or rods slapping the crank if the engine wasn't primed.
As far as timing I always set the timing up for first time starts to 6/8 degres before TDC. Also prime the fuel system You want it to fire with in the first couple seconds.
Tim