We recently had our kitchen remodeled and that included adding a microwave over the stove. The contractor vented the microwave/range fan above the unit through a cabinet up through the attic to the outside. The problem is that every time it rains the sound of the rain hitting the vent tube is so loud it sounds like it is right in the cabinet above the microwave/stove. I checked for leaks but did not see anything (so far!) The contractor put some insulation around the vent tube in the cabinet and also in the attic however the problem still exists. Is there a certain type of vent (such as galvanized steel) that should have been used or is there something else you can suggest so that the sound of the rain doesn't echo into the kitchen. Thanks for your help.
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.