Bro, common hai, as these smartphones supports Dolby atmos(Flowing sound/or you can say acoustic sound technology) and speaker sound is quite high..so it results in the vibration of back panel.. You can feel it more clearer when your device is connected with external speakers..Its like sound flowing from entire device.. Even, I feel the same in my S10+, when volume goes up like more than 80%..so, no need to worry about it..As sounds flow from the entire device so it feels like vibration at the back panel..if vibration is really high, then you may visit service center..but in my opinion, just relax..as it's design feature
NOTE: LG/HTC HAS THE SAME TECHNOLOGY BUT THEY HAVE FAR SUPERIOR DESIGN AND THERE IS NO SUCH ISSUE WHILE PLAYING MUSIC, WHICH IS QUITE IMPRESSIVE..
