try this, when it starts vibrating, put your thumb on the center of the grille and apply pressure
mine used to do that, it was the metal part of the grill that was vibrating against the plastic circular frame of the grille
if you look closely, there is some type of glue on the inner circle of the cover that is meant to hold the metal grille in place
i don't think that does the job properly
on the HIFI setup, there is a thin fabric cover between the metal grills and the plastic frame
on the enhanced, there is no such fabric.