We're having this problem too since we had our old furnace replaced by a high-efficiency one. The installer said our fan is on its lowest setting, but it's still difficult to hold a conversation in our living room because of the hissing floor vents.
There have been a couple of references here to noise being caused because the installer "economized the install". What does this mean? Did he omit some kind of equipment that would've given us more control over the airflow?
Thanks for your help, as this is really annoying! Cheers, Ander