Two identical ervs-one is silent the other is loud – GreenBuildingAdvisor

I have two Panasonic intelli balance 100s,

They are direct ducted to the bedrooms. 

1 serves 4 bedrooms and is silent- there is no way to tell it’s on. This is served by 6 inch insulated flex duct

The second one serves the main bedroom via 16ft of 6inch flex duct (uninsulated)and is annoyingly audible.

I can’t figure out why it’s loud.

Would changing to insulated flex fix this? Is the fiberglass a sound absorber?