I've noticed that with my QS8s from time to time. It isn't internal to the QS8 for me, and I don't *think* that it is the grills, but that the speaker is vibrating against the wall itself. I just haven't taken a few minutes to put some foam tape (strips of foam with and adhesive backing to it) on to the wall bracket to reduce the ability for the speaker to come in direct contact with the wall.
I guess it is just that the QS8s can handle so much sound that this becomes a problem. I would rather have this (fixable) problem than have crappy surround speakers.
Just my thoughts.
