When you are happy and humming your favorite song or whistling it is a normal act. People use whistling either to get attention or call out to someone or just to entertain themselves. But in many cultures whistling, especially whistling indoors is associated with bad luck or evil.
In Turkey it is widely believed that whistling at night is calling for the devil. However, this particular superstition is not unique to Turkey but can be found in many other cultures around the world. In Japan, people believe that whistling at night might disturb the quiet of the night and thus bring danger to yourself. It is the same in China, but they do not believe that whistling is only a call for evil spirits. Skillful Chinese yoga practitioners believe that they can summon supernatural beings, wild animals and weather phenomena with a whistle.
In Russian and other Slavic cultures, whistling indoors is superstitiously believed to bring poverty and there is a Russian proverb saying "whistling money away." However, unlike any other cultures, Russians do not believe that whistling indoors or outdoors makes any difference. In Estonia, it is also widely believed that whistling indoors may bring bad luck and therefore set the house on fire.
In the U.K., the whistling superstition is taken to whole another level. In British culture, there is a superstitious belief in the "Seven Whistlers" which are seven mysterious birds or spirits who call out to foretell death or a great calamity. In fact, in the 1800s, hundreds of mine workers stop working for a day, claiming that they heard a mysterious whistle and could not work as something bad would happen.
All these superstitions are probably linked to the times when people were pagans and were highly concerned with spirits and the unnatural world. Today, most people do not even know why they are against whistling; yet they are nonetheless, following in the footsteps of their ancestors.
Apart from cultures, certain professions have superstitions about whistling as well. For example, whistling backstage in a theater is considered a jinx. Actors believe that if someone whistles backstage, bad things are on the horizon not only for them but for the entire company. Sailors also ban whistling on board, believing that the whistler draws bad luck to the entire crew and the ship.