What decibel range would be considered a "quiet" range hood?
From Lowes.com: 

"Range Hood Sound Levels

The sound that a range hood makes is measured in sone. One sone is roughly equal to the sound of a refrigerator running. Normal conversations take place at about 4 sones, and light traffic rates up to around 8. Use sones to compare units, but be aware that the higher the CFM, the higher the sone rating is likely to be. Look for normal sone ratings to find the quietest hood at normal operation levels."

Not all online sources publish sone specifications, but you'll often find that number on the actual packaging.