After looking a number of purifiers, I eventually relied on Consumer Reports for guidance and settled on the PURE 311 Auto.  We have the 311 in our master bedroom.  The unit is quiet at low and although a bit loud on high, we run it in auto and it has never run the speed up.  We have experienced a significant improvement in breathing and nasal stuffiness since placing the 311 in our room.  It just keeps the air clean!