My startup makes small (2.5lbs) electric helicopter uavs. At 50ft you can't hear them any more. If you are inside your car, I could position the heli within 5-10ft of you before you might even notice.
What's your startup?
You could make reasonably-sized miniature zeppelin drones, which can be made quiet and stealthy, but I'm not sure how maneuverable they'd be in a stiff breeze.