Well whatever you do don't let the audience know what you're whispering, I mean sheesh.
By keeping the information being whispered from the reader, the reader can't mistake the point to have something to do with the specific message. The message is unimportant; the joke is in the communication.
Three characters are left out of this strip. We can figure for ourselves why Snoopy and Linus aren't included. That Schulz shows preference for Schroeder over Shermy just goes to show how already poor Sherm is kind of a second-class cast member.
Odd rounded frames on the panels here.