The tires develop a flat spot overnight. It's noticeable because the sidewalls aren't very tall and the suspension is on the stiff side. As you drive, you work out the flat spot and the vibration goes away. You may also notice the vibration more at a particular speed range. In my case it's in the neighborhood of 40mph.

I've spoken with some "car people" who've told me that certain tires, and older tires, are more prone to it than others. I don't know the reality of that statement yet as I'm still on my original tires, so maybe someone else can address it.

You'll find other threads on this subject on the forums if you want to check them out.