The Morning Seismic Shock

Every weekday morning and some Saturday mornings, at very close to 6:30, there is a loud noise that sounds like the clatter of metal out on the street.  It is followed a fraction of a second later by a vibration that shakes the walls of my house and feels much like a small earthquake.  I presume it is a garbage truck driving at a high rate of speed over something buried underneath the road.  But it gives me just an instant of scare anyway.