* -er: whisperer (a person who whispers)
* -ing: whispering (the act of whispering)
* -ed: whispered (past tense of whisper)
* -y: whispery (characterized by whispering)
* -ing: whispering (the act of whispering)
* -some: whiskersome (likely to whisper) - This is less common but could be used in a creative context.
There are other suffixes you could add, but these are some of the most common and meaningful.