I have metal circular duct work in our residential units. They are VERY loud. We have reduced the speed on the air handler and it helped a small amount but not enough.
I have been given two suggestions I wanted to see if anyone had a opinion on;
1. Insulate the duct work. I actually have seen this done in a renovated school project where the ducts were too loud and condensation occured. It worked perfect.
2. Try DuctSox or some other inflatable duct system. I've read a few post on these and that does not seem to be a good solution.