Great question and sorry for not being more clear, the speaker grill rattles when I drive over bumps in the road.
So, you can't trim some small pieces air conditioner foam and place it under the edge of the grill without it showing?
I had to shove some foam up underneath my panoramic sunroof headliner to keep it from ticking and clicking while just normally driving over smooth road.
I also have the rear window rattle. And I'm extra lucky enough to have a ripping sound in my dash intermittently upon stopping and starting. Apparently something flexes enough to sound like Velcro fastening and unfastening. Or else it sounds like a bolt rolling around. It almost sounds like it is coming from near the center speaker grill as well. My Longhorn has the HK system.
Possibly you could try to lift the edge of the grill up just enough to shove some foam there to put upward pressure on it and keep it from rattling, maybe do it on the windshield side...
I always had bad luck in the past taking a vehicle to the dealer to fix rattles. The mechanics knew less about how to fix the rattles than I did and damaged interior trim. I just figure out how to fix it myself now.