How can I reduce the sound of rain on a range hood vent?
First, the vent exit should have a weather flap on it that should mitigate sound somewhat. Be sure that it's present and functioning properly. 

The sound is echoing through the duct, so insulation outside the duct won't do much. You'd need insulation inside the duct to do that, which isn't a good idea here. 

Instead, protect the vent cap from direct impact by rain. Install a secondary cover of some material other than metal so it doesn't ping as loudly.  You might get away with just wrapping the existing vent with self-adhesive rubberized roofing membrane to make it heavier and softer.