Well, I take that back. If they have a gravity generator, perhaps they have a way to keep acceleration forces inside down to 1G, and if that's the case, the gravity generator would also be its propulsion system. Maybe it compresses a dense mass very quickly in a pulse, which sends out a gravity wave, and they are able to surf that wave outwards, keeping G forces inside at normal levels. It could explain the hum often mentioned. It's not an audible one, but one felt through the manipulation of gravity around.