UFOs are aways silent, outside. UFOs don't have noisy jet engines. How quiet are they inside? In atmosphere, do occupants hear wind sounds from air on the hull? Or are they in a 'bubble' that excludes air? If they exceed the speed of sound often, they would normally hear or feel the effect, But I suspect the occupants are shielded from that. Electrogravitic propulsion probably comes from rotating machinery, but it should inherently not be noisy with random noise. Perhaps any sounds inside a craft are more from rotation in machines with its periodic kind of sound. That would be heard as a tone with some repetitive waveshape. It could be a hum or a drone but not a shrill or jarring noise, because that would be hard on crews if they have ears. We are going to assume that aliens do have both sight and hearing, and maybe other senses. Energy sources might be nuclear or free energy, but not internal combustion nor jet engines. A UFO probably sounds like a submarine inside.