Apple - Turn off buzz when plugging in iPhone
- Update to iOS 9.
- Go to Settings, Sounds, Vibrate on Silent and deactivate it.
Plug in your head phones into the phone, turn off silent mode and turn your headphones volume down.
You may also want to turn off vibrations in sounds then charge your phone! It will make no vibrations and if you put it back on mute, your phone won't vibrate as well :)
There is no way to natively disable this other than turning off vibrations completely. Only your iPhone does this because the iPad (and the iPod touch) don't have vibrate motors.
If you jailbreak, there is a tweak called PowerSoundDisabler. It's a simple tweak to disable sound and vibration when respringing or plugging in via USB.