Were there any large trucks driving in front or around you? They can get little rocks under their wheels, which then get launched into the air at top speed, which can then hit and break windows. That happened to my window--it was a earth shatteringly-loud noise from a tiny pebble that a big semi threw into my window--the window didn't shatter, but it has a big crack in it now...it is so unnerving, though! :wtf2