Do deaf people who use sign language whisper by lowering their hands? Holding their hands closer to their bodies? That would work. I was thinking my friend who works from home could teach her children sign language so they would be able to talk to her while she's working without actually disrupting her. But I decided that would still be a real interruption - they would expect her full attention since they weren't making noise.
Oh, well, it was a thought.
But I do wonder if deaf people "whisper". It's so cool to see people signing, I've learned a few signs, my cousin learned a song in sign to show two deaf people at his church.