In Athens, Georgia 1974, I had a huge triangular-shaped craft fly over my car, just above treetop level. No sound at all. Later, especially since we live near Whiteman AFB and see the Stealth occasionally, I've wondered if it was an experimental version of that as the Stealth has the same basic shape. But I think that unlikely because
(1) The Stealth is called that because it can fly under radar: it's as far from silent as you can get. Even when it goes by at a high altitude, the whole sky seems to vibrate; and
(2) Even speculating that this was an experimental version in 1974, it would be quite out of character to be testing it out right in the middle of a densely populated area.
I'm quite comfortable with the idea of intelligent life elsewhere in the universe, although IMO they'd have to have far more sophisticated technology than what exists on Earth to cope with the distances involved.