Whistling is one of the most powerful yogic techniques available to us, even though it was not discovered in India, and as far as I know Yogis are not noted for whistling tunes.
But, what happens when I whistle? First off, I stop worrying, all that mental chitter chatter fades into the back ground. This is one of the great goals of Yoga, the cessation of unwanted mental activity. Whistling also involves long exhales and short inhales, eliminating Apana and creating mini Bandhas in the upper and lower abdomen, including the anal sphincture , all desired practices in Yoga.
Lastly and not least it makes people around me happy when I whistle, and it makes me happy too.
So if you are seeking mental peace, therapeutic breathing and desire to make people happy, whistle, like a bird!