How loud are 80 cfm duct booster fans?

The ones at Home Depot, 4". Are they quiet?

depends on the quality of windings in the fan and how they are mounted.

ask the HD people to hook one up for you and find out.

Dude I just bought a 6 inch 250 cfm and it's really quite...I can barely hear the damn thing...great price, the only problem is that you have to buy the wiring - it's direct wiring...

i have a 750 cfm and its loud but not to the point where its a problem. i think
an 80 will be almost silent

You can put he 240cfm in a box and run the ducting through the sides, pad the box and it is very quiet. 80cfm ain't gonna do much for you and the 240cfm is only like $10 more.