Against my better judgement, I'm gonna make a statement on this subject. I am a certified small UAV pilot for the Army. I'm qualified to operate the Raven and Puma series of UAV's. It is a normal part of our training to practice following cars and individuals as it is a skill routinely employed in the theater of combat. The current version of the Raven makes a buzzing sound when flying, but they're trying to fix that with different propeller designs. The Puma is very very quiet. The larger UAVs, like the Shadow(it's about the size of a Volkswagen Beetle), are exceptionally noisy and can be heard from a long way off. The Shadow UAV uses a snowmobile engine for power and as a consequence it sounds a lot like a pissed off chainsaw on steroids. Also, for all you paranoid folks.... There's an offshoot of the Puma program called the Switchblade. It's basically a man-portable, remote control warhead. I actually got to spend some time with the program manager for the manufacturer and it NEVER missed it's target even in the prototyping stages. It is currently in use with SOCOM and the CIA.