Three unlit triange shapes moving slowly in formation 500ft above with little sound

At approx 1:00 AM of the summer of 1981 I saw three trianular shapes, no lights, traveling in a triangle formation traveling West to East while sitting at my back door. There was no moon and the area where this occured is rural so ther were no city lights. The shapes made a faint hissing, whooshing sound as they passed over. They appeared to be about 500 to maybe 700 feet above me and and appeared fairly large. They past over head in front of me moving slowly, there were trees to the North, East and West of me with my house to the South so I only saw them for about less than 10 seconds