I have this issue! I am using the third Lumia920 (first one bought on the day it was released and exchanged in couple of days and second one exchanged through AT&T warranty division for third one). All of the phones had/has issue with this and its frustrating. Today, the representative from AT&T warranty division called to see if I had received my third phone and she asked me 'are you outside in breeze?' I said "if you could hear that its ok, but if I am also hearing it, its a problem!". Not sure if it's the AT&T network or the handset? If I sit in the car and turn the AC/Heater on, the blow of wind through the vent can be heard on the phone! Strange, never happened with any of my old phone!
I am confused! I want Lumia 920, but if this problem is not fixed, I am going away from Nokia for the rest of my life. I feel sad if I do that, but I need to move on!