Would they really be silent? Ion thrusters are very efficient because they accelerate particles to high velocities (typically 30 km/s exhaust velocity, up to 8

Would they really be silent? Ion thrusters are very efficient because they accelerate particles to high velocities (typically 30 km/s exhaust velocity, up to 80). They're low-thrust because they do this to a very tiny quantity of them—nanograms per second. This is fine once you're already in orbit and you can plan your maneuvers and a 3-hour burn isn't a big deal. Compare this to a typical kerosene rocket exhaust, ~3 km/s velocity but much more of it, over 2,500 kg/sec of fuel flow in the extreme cases (e.g. Apollo F-1), and maybe between 0.5 and 10 kg/sec for a passenger jet engine in atmosphere (very efficient because it doesn't have to carry its own oxygen). So it seems to me if you've got enough thrust to fly a plane, even with a very gentle impulse, something that's almost a glider, it's still going to make some noise. Probably very high frequency, maybe high enough that it won't be audible. If you're hauling people or cargo, then I think it would be as loud as any jet.