A new experimental YouTube feature reported by some users allows anyone to hum or record three or more seconds of a song to try and identify it. “If you’re in the experiment, you can toggle from YouTube voice search to the new song search feature, and hum or record the song you’re searching for 3+ seconds in order for the song to be identified,” an official support document for the new test reveals.
Source: YouTube Testing New Song Humming Search Feature on Android