I am really satisficed with the Clorox Medium Air Purifier that I got for my master bedroom. The majority of the time, the fan is so quiet I can hardly even hear it. Even next to my bed at night, it is quiet enough that it does not disturb my sleep. 

The only time the fan noise is noticeable is when there is a noticeable about of cooking smoke in the air if, for example, I leave the master bedroom door open when searing something in the kitchen. Once the extra particles are detected in the air, as indicated by the air purifier's display, the fan increases in speed until the air quality return to it's "normal" level.