Hi
I have that vibration too. (sometimes)
The vibration happens when the volume is near max specially with "bass" sounds. I think isn't nothing wrong/broken etc..
Is like when you turn up very high the volume in car, home speakers or even more if you use an high "bass" level, amplifier or something like that in car or every where speakers.
Try maybe to decrease "bass" level in the stock (mini) equalizer and search the best level 4 you.
Sent from my SM-G903F using XDA Free mobile app
For me this vibration happens even when the volume is like 30%. Or when I am talking with someone on the phone