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I think someone just unplugged you from the matrix...

Seriously though, that's a crazy noise, that was immediately after unlocking your phone?
 
Mine did the exact same thing! It would make the sound every time any sound was made (such as typing or unlocking). I think it might be a bug with an app that caused it.
 
That's sounds like a cicada's mating call. It's been really hot lately. Maybe one took up residence inside your iPhone.

If there's an Apple store near you, take it to a Genius Bar. I'm curious what's causing it.
 
transformers-john-turturro-jon-voight.jpg


"So far, our only lead is this sound ...."
 
Mine does that too on certain screens (also when unlocking it) and if a "gray" overlay comes on the screen like when a pop-up menu or a notification appears, but it's not that loud. You have to be in a quiet room with it close to your ear to hear it.
 
Mine did the exact same noise when I locked the screen. I restarted the phone and it hasn't done it since.
 
Mine did it while receiving a text message a couple of times. A reboot fixed it.. for now.
 
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