Hearing a loud sound while on FaceTime

I'm on FaceTime a lot and the last few weeks I've been hearing a loud, high pitched tone while on FaceTime with my friend. I think it's coming from his end because the sound goes away when he unplugs his headphones. We thought it was the headphones but he bought new ones and it's still happening. He has a 5s, I have a 5c. Any idea what it could be?