6-8 white lights in a straight, horizontal line, moving very slowly west to east above the Missouri River near Malmstrom AFB.

Observed 6-8 white lights in a straight, horizontal line, moving very slowly west to east above the Missouri River on the east end of Great Falls, MT near Malmstrom AFB.  Lights remain fixed relative to each other, as if on one large object.  We see plenty of planes here and it was not a plane. We observed for about 3 minutes until the lights disappeared into a large cumulus cloud.  I did not hear a sound but my spouse did and says it was a faint humming sound.