Humming to yourself is traditionally a sign of contentment and inner ease. Dream books read it as comfort you create on your own, or a tune of memory you cannot quite place.
Where in your waking life have you met something like this lately?
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.