Maybe I’m venturing into tinfoil hat country, here, but I’m pretty sure I once experienced a flyover by a stealth helicopter. I was camping at a lake in central Texas, during the Fall of 2003. Everyone else had gone to bed, but I was unable to sleep and was sitting up by the remains of the campfire, around 2 AM, just listening to the sounds of the forest, when I very clearly heard a distinctly unnatural sound pass across the dark sky overhead. It was very quiet, and very slow (rhythmically), but unmistakably a helicopter: whup whup whup whup whup. It was a clear night, and the speed at which the sound passed overhead meant it had to be flying at low altitude. There were no lights, just the sound, and I had a very eerie mental image of the glowing silhouette of my body, sitting beside the bright star of the cooling campfire, on a thermal imager cruising somewhere through the blackness above.
This wicked-looking rotor design is called “blue edge.” You can read more about it, and hear a comparison to normal rotor noise, over at Wired’s Autopia.