Hey all--

I have a mystery bird, but very few details, maybe not enough to go on...

A couple of mornings ago, a large bird swooped over my car and into a
neighbor's yard -- my first thought was raptor.  By large, I mean
Pileated-sized.  It flew low over the yard, glided, pumped its wings a
couple of times, and headed back into the trees, much higher.

The thing was all gray on the back, and all I could see were distinctly
white tail (coverts?) feathers on the outside of the tail, which was
otherwise the same color as its back.  The colors were clearly delineated. 
It reminded me of junco coloring.

Gos?  Any other guesses?  The mystery is bugging me...