Rear View Mirror Vibration

Finally taking a long road trip with my 2015 GT. I noticed when I am cruising down the Interstate the view in the rear view mirror is a bit blurred. When I hold the mirror, it clears up.

Has anyone else experienced a vibrating rear view mirror at speed (i.e., 70 mph or so)? If this is not normal, is there a screw/nut that can be tightened to eliminate the vibration? Or is it just a function of the road-smoothness you are on?

Thanks.

In my case it was several of the tires being just slightly out of balance.

When I brought my car in to have my GT's pads and rotors changed due to the TSB I asked them about my loose vibrating mirror. On my service sheet it says it was loose and they adjusted the bolt. So I'm assuming there is something.