You need to put abnormal pressure on the back with a white screen present to even attempt to reproduce the distortion.
As for the sound, I use a smart case and don't feel them, but then I also mainly use headphones when using my iPad to watch video or play games.
Do you think it is normal that I have to buy the smart case (79 Euros) or to use the headphones to avoid the vibrations? About the screen I'm talking about the back lite problems (like less light near some borders).