Product review (3 stars): Moderately Loud Compressor
There is a moderately loud hum during the short infrequent periods that the compressor operates.  It is tolerable to me, but this was an unpleasant surprise - especially since my decision to purchase it was based in part on another Amazon reviewer's comment stating it was quiet!  I'll keep it but probably would have looked harder for a quiet unit if I knew this before my purchase.