A friend has a engine that upon firing for me was making a good bit of rattle...It was cool out and the engine was cold. This was a low deck short rod engine BTW. After warming it wasnt near as bad...Im pretty sure it was piston slap we was listening too. It was just noisy, kind of like a 24V cummins in the morning...by the time you get to town its not near as bad as it was.