knocking or tapping on outgoing calls

I have the same problem. Android and so far only happens when I call a specific person who lives in another state. It's a rhythmic knocking and it is on and off throughout the entire conversation. I saw the apple support page as well and it's clearly only happening to T-Mobile clients. Obviously people first think it is the phone itself before realizing it's actually the cell provider, which is why that apple thread is full of complaints. We need those same people to come over here and fill up this thread in order to get T-Mobile to listen and take the matter seriously.