Diesel sound

If pistons are put in on the loose side they will make noise. A forged piston needs more clearance than a cast piston because it expands when the engine gets up to operating temperature. Forged piston motors are a little rattley until they get warmed up. If the machine shop was a little overzelous with the clearances this could be the noise.

One other thing there is an old trick of putting pistons in backwards so the piston will dwell at top dead center longer because of the offset wrist pin. The motor will make more power but will also make more noise. When you had the head off you could have found out if the pistons were in backwards because the notch or arrow would have been pointing toward the rear of the motor instead of the front.

If the piston noise is light I wouldn't worry about it. But if its pretty loud it means you pistons are really rocking in the bores and you don't need that.