General:
* Murmuring
* Hushing
* Muttering
* Sussurating (formal)
* Whispering (can still be used, but adding an adjective like "softly" or "quietly" emphasizes the softness)
More specific:
* Breathing (suggests a very quiet whisper)
* Rustling (suggests a whisper that's almost like a gentle sound)
* Purring (suggests a soft, comforting whisper)
The best word depends on the specific context and the nuance you want to convey.