Why does my earpiece speaker buzz when the other person is talking?

I am on my third iPhone 6 plus now. On phone calls, when the other party speaks, their voice is accented with a slight buzz or rattle. As if they were speaking through tin foil. It drives me nuts. My first iPhone, the apple store tried to replace the LCD screen, which houses the earpiece, and that didn't work. They replaced my phone. The second iPhone worked for a few days and then the noise, buzz/rattle, started on this phone. I took it back and the apple store staff could clearly hear it. I am on my third iPhone 6 plus, and two days after bringing it home, i started talking to my wife, and low and behold, this phone started having the same noise. Do i really want to take it back to the apple store? I'm tired of this problem and am tired of going to the apple store. My wife's iPhone 5s sound great compared to mine. Help!