1. v. To make a clear musical sound, a series of such sounds, or a high-pitched, warbling sound by the forcible expulsion of the breath through a small opening formed by contracting the lips, or through the teeth, with the aid of the tongue. (dictionary.com)
a.k.a Something that is fun to do by yourself to entertain yourself, but annoys anyone else nearby.
Joe: "Let's not sit over there, that guy is whistling and it's really annoying"
making any non-verbal noise through the mouth by means of passing air in or out over teeth and tongue
That guy is trying to get peoples' attentions by whistling while he works.