Warm late summer-time, my roommate (both in the USAF) & I heard a sound in the sky, approaching from the west, passing us overhead, then on to the east.

Early morning at 1:30am, having had some brews, very humid, hot and sitting on the ground in chairs.  We watched the object  pass directly overhead.
There were no navigation lights.  It was loud and sounded like somebody creasing a thick piece of one-ply cardboard.  The craft was about 500ft up.
It sounded like a jet, but not like any I had heard at multiple air shows I'd seen in the past.  We were members of a SAC unit that was a part of a GSU (geographically seperated unit) approx.  120 miles west of Omaha, NE.  Our mission was in part support of SAC by operating a VLF transmitter site there.
We both agreed that we did not see any exhaust or visible light from the craft.  There was very little light in the upper clear sky as the moon was hidden by low partly cloudy cover.   
About 15 min. after the sighting, I smelled something like jet exhaust that I have smelled before at airshows or on a flight line.  On a note, the site we worked at had a radio tower, the height of which was a little higher than the Empire St. building in N.Y.  This tower was surrounded by "HAZ" markers at the four sides of our 1/2 mile sq. site.  (HAZ was short for HAZARD and lighted at night)  Our site covered 4 corners of four counties, not unlike the "four corners".
Years later and keeping in touch with my friend, I saw an airshow at Scott AFB, IL, in 1991.  The very same shape and sound was emitted by a craft that flew overhead: the F-117!   As it turns out, my supervisor knew that the F-117 would do flybys by remote USAF sites in the midwest in the late '80s!  From UFO to IFO.
Date is approximate.  PD