You might have a vibration somewhere you're not aware of. My mirror use to vibrate but the truck felt fine. I replaced my U-joints one day and for the heck of it had the drive shaft balanced. My mirror stopped vibrating after that.
Also, one day I started up my freshly rebuilt 270, the mechanic and I checked everything over and I was about to back out of his shop and take it on its maiden voyage. As I looked in the rear view mirror I noticed a barely perceptible intermittent blur. It couldn't be heard or felt, but you could see it in the mirror. Long story short; the engine was bad and I have a new mechanic.
The point is; the slightest vibration in the engine, drive shaft, wheels/tires, etc. is exaggerated in a mirror because you're not looking at the mirror you're looking at a point sometimes half a mile away.
