sound travels through vibration didn't you studied on physics that's normal when the speakers are loud the sound creates vibrations patterns to reach to our ears that's normal nothing to worry too much
I got ur point dear. I was comparing my note 9 sound vibration while playing media on youtube with my another note 9 in my family. Actually on max volume level, vibration on my note 9 is more than other note 9. Also I had given my note 9 few days back for battery change in service center. Post that I m feeling more vibration in back panel. Can this be related to teardown done on placing new battery and while doing that speakers placement effected or something is touching the bottom speaker n creating vibration. Wild guess....any suggestions dear?