Do you have any history on the engine? If the pistons and/or rods were installed 180 degrees out ("F" mark toward the back) the engine will rattle like a diesel- - - -piston slap due to the wrist pin offset being in the wrong direction. If the rods are numbered, the numbers should face toward the camshaft. That's easy to check by dropping the oil pan.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!