To dream of a whistle represents a timeout or pause. A decision to take a break or stop certain behavior. Telling yourself or someone else when enough is enough.
Example: One person dreamed of seeing a red whistle being thrown away while a white whistle was putting put around their neck. In waking life they were beginning to take more breaks while doing very stressful work. The red whistle represented the stress and negative emotions associated with never taking a break and the white whistle represents a more balanced approach to getting away from work when it was getting to be too much.