Noise damping of metal is accomplished using pressure sensitive adhesive backed damping sheets or paint damping using a brush or sprayer, adhered to the surface of your metal. The thickness of your damping compound should be the same thickness as your metal, and have about 80% coverage of the metal's surface. Damping takes the ring out of metal like the ring out of a drum set cymbal.
Antivibe® DL-10 Metal Damping Compound
Soundamp Damping Sheet