If the sound is only present on phone calls and not their at any other time your problem may be a Network or weak signal issue. Try using the phone in different calling areas and also see if the sound is their when using the earphone.
I have been having the same issue. Its as though the person I am talking to and I are talking through some sort of metallic barrier, for lack of a better description. It has happened while on an HD call and while on an analog call but with the Advanced Calling 1.0 turned on. I have heard it through my handset earpiece, the handset speaker phone and through a speaker connected with an aux cable. All that being said, I have done some troubleshooting on my own and noticed that this issue only happens to me when I am using the device with the Advanced Calling 1.0, or HD Voice, option on. When I turn off Advanced Calling 1.0, it goes away. Do you also have the Advanced Calling 1.0 feature turned on when you have this issue?