On the iPhone 5, with headphones, you can hear a slight static click just before a sound it initiated-keyboard opens, music starts playing, etc. If you are on contacts and click the keypad tab(switch between keypad and contact tabs is very evident) it also happens. You can also hear it quite well from the going from the home screen to spotlight search and back-of course mute has to be off and keyboard clicks have to be enabled. Tried 3 different iPhone 5s, all there. 6 different headphones, all there. 4S has it but much less pronounced. Seems to be the audio amplifier turning on/off. I would have thought they could have eliminated the static popping noise a bit better.