In the venue I do sound in, our lighting guy brings in custom built robo-lasers for some shows. The servos whine is right at 8kHz. It drives me nuts when he fires them up because I always think it's the PA getting ready to take off. It also doesn't help when they're mounted on the truss right above the drum kit and the overheads pick some of it up to. He said in the next redesign he'll be using different servos and applying internal acoustic damping. I can hardly wait!