Whispers are secrets, rumors or false promises/lies told in hushed tones.
The contents of certain magical volumes are so powerful that they remain secret to even the majority of serious magicians. The books themselves are often magical in their own right, and are so rare that their very existence are doubted by the very few that have heard of them in whispered fable. It is whispered that many of the formulas in The Sorcery of Old were written in languages unknown since before the ascent of humanity.
One can hear the name of the Land of the Green Isles whispered in roadside inns off dusty roads from the hills of Daventry to the sea of Tamir...
Sometimes whispering is used to show silent respect. Graham whispered a short prayer of thanksgiving at the Monastery of the Blessed Wilbury after hearing about the Magical Doorways from the monk Fragola.
The evil Witch of the Dark Forest made noises of cackaling and whispered mumbling under her breath.