If it sounds like all eight knocking like a diesel, it's piston slap. If it has forged pistons in it, that's all it is and it's normal. All modern forged pistons and some high end modern cast pistons are "cam ground". That means at room temperature they are not round, but oval. They have excessive clearance when cold, so they rattle. Once warmed up, they expand and get round and the clearances get right and the sound either diminishes greatly, or goes away. You're probably spending money for nothing.