Heard odd sound, saw triangular craft, three red lights, flying low as it passed overhead.

Left my shop walking to my house and heard a unfamiliar off and on swishing sound, looked up and saw a triangular shaped aircraft gray metallic color, three red lights centered underneath and one white on each outer point. It didn't make a jet engine sound but a slow pulsing sound that was I was almost unable to hear, I watched come and go over my land in a straight line. When it passed over the tree line I couldn't hear any sound at all. I had my phone but was so surprised I didn't even think of taking a picture. not sure if I saw an experimental military aircraft or what but glad I saw it.