When my phone's screen is off and I push and hold both volume buttons, my phone will vibrate one or two times, but I can't tell what, if anything, actually happens. It doesn't change the phone's volume, it doesn't change the phone from ring to silent or vice versa, and it doesn't always vibrate. Has anyone else tried this or noticed this before? After some other playing, it will also do this with the lockscreen showing. Any ideas?