I was in my backyard at 2am sitting on the step of the back door which faces west. smoke break. i heard a very faint hum. less than 1 second later i looked north and saw a black triangle, altitude about 500 feet. white circular light in each corner. the dark black triangle was more prominent against the dark gray sky because there was slight cloud cover and the valley is populated and well lit. it flew from north to south. there is some military aircraft seen around here. this was definitely a military aircraft. i would normally have thought... "cool... they've got themselves a new toy they're checking out at the local test base i think is somewhere southwest of here." the only reason this was different was because the speed + size + sound did not compute with my remedial high school level of understanding physics. it flew across the valley in less than 2 seconds. size was probably 40-50 feet corner to corner, 400-500 feet above me, approx 400 mph. i should have heard or felt something more being so close. the hum was so faint, i doubt most people could hear it. "stealth" was my immediate thought. after thinking about it for over a month, i'm also a little ticked off i had to see something like this. i could have lived my whole life happily without this enigma. but i'm sure, one day in the distant future, the declassified info will be common knowledge and i'll be reminded of this 2 seconds of my life.