BTW= By The Way. The metallic rattle noise is most probably detonation. Detonation will result in excessive stress and heat in the combustion chamber. Usually the piston is the weakest point and detonation can result in melted piston domes. I've had the melted piston smear down around the rings on gas engines resulting in loss of compression and actually made dime size holes in the piston heads on gas engines as well. I've not personally seen diesel engines damaged from detonation, but I assume their prone to it as well. I assume if it is running fine you probably didn't hurt it, I only wanted you to know that it might get hurt if it was allowed to detonated for too long.