The Gaian Gunboat engine (the one I tested, but I assume it's the same for the others) has the same idle sound whether on or off. As in, it sounds like it is still running when you engine kill.
The rumble is part of the loadout of the ship, so even if we swapped the rumble to a different one that is less loud, it would require repurchase of the ship.
I will swap it to the same rumble as the Gaian Gunboat, which is sfx_rumble_gunvessel, likely quieter.