why my iphone buzzing

I was using FaceTime when my screen when black, I then shut it down and plugged in to the charger and it just started buzzing.  I couldn't get it to stop.  I tried holding down the home button and sleep button and it just kept buzzing.  I let it die down and tried pugging it in again and got nothing but more buzzing every time I plugged it in.  I am sending it back to Apple.