Purchased the unit a month ago.  Low mode is whisper quiet, medium light fan and high is noticeably on.  Unit can be put on auto which runs low most of the time until the sensor picks up more air pollution, in which case it goes to high.  Design is well thought put with the entire base being a return pulling in air and blowing out the top.  Fan is solid, no mechanical defects, runs smooth.  Unit cleans the air, has been a long time coming since an actual product is made to last and does what it is supposed to do.  Cant go wrong buying this unit.