Stirlings are external combustion engines, and can burn whatever fuel is available. That takes care of requirement number one. Steady continuous combustion doesn't make any appreciable noise, that satisfies requirement number two. The extreme torque reversals place severe constraints on propeller design. No torque reversals, no need for a prop that must also serve as a flywheel, and silky idling at very slow speeds.
Smooth power can save weight in engine mounts and isolators, too.
Heat and cool. Heat and cool. The same chunk of gas is alternately heated and cooled. Heating a trapped body of gas raises it's pressure, and the pressure pushes a piston which does work. Then the gas is cooled, and the piston returns.