Okay this is a bizarre issue that I think affects most HTC One devices and people don't even realize (unless you have incredible hearing like myself haha). Anyway, put your volume up and then do anything that makes the speakers make noise...doesn't matter what it is. As soon as the noise stops, put the speakers directly up to your ear and see if your phone makes a popping sound. I had this issue on my device (I was actually able to hear it when I was alone at home without putting the speaker up to my ear) so I checked out both display models at AT&T and they had the same issue. Does your device make the popping sound?