Technical note from STMicroelectronics about popping noise usually heard on audio amplifiers and how to minimize it. Link here (PDF)
Pop and click, or rather, the absence of it, is a characteristic that makes a lot of impact in the world of audio amplifiers. This is especially true for those destined for headphone-equipped applications (such as mobile phones and MP3 players).
Pop and click are the names given to the popping noise that may be heard through the headphones when you switch on or off portable audio equipment or mobile phones. The noise is generated by a voltage difference at across the output stage of the amplifier at switch-on or switch-off before it reaches its idle (or equilibrium) state.
from Dangerous Prototypes http://ift.tt/2hSV27q