Vibrations from the soundaktor was fairly common in the mk7 GTI and quite a few posts on this over the years. With the mk7 GTI, many owners disconnected the soundaktor completely to stop the vibration noise. Don’t know if it can be disconnected in the mk8 / mk8.5 though; I might be wrong but I seem to remember reading it can be set to zero in the mk8 / mk8.5?
One of the first things I did in my 2020 Polo GTI+ was to set the engine sound to ‘Eco’ in Individual mode as I dislike the artificial / fake engine noise. As you’ve said, no vibration noise from the soundaktor when in Eco.