A whistle is read as a signal — a warning, a call to attention or a demand to stop. Blowing one yourself is traditionally linked to speaking up about something wrong.
Does the traditional reading fit your dream, or did it mean something else to you?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.