In the kitchen scene, it's clear that Tom and Daisy are furtively whispering but it's unstated what they are speaking about. Given the context though, it is pretty obvious that they're talking about the car crash and the death of Myrtle. Tom and Daisy are conspiring against Gatsby in a manner of speaking because they are working together to spin the story of the car crash. Daisy needs protection from this story and Tom is just the type of amoral person to do it.
