it can be. If you can operate a thermodynamic engine at a single 'operational point' meaning fuel/air flow in and energy out then you can heavily optimize its e

it can be. If you can operate a thermodynamic engine at a single 'operational point' meaning fuel/air flow in and energy out then you can heavily optimize its efficiency in terms of thermal efficiency and noise produced at that specific point...or at least a few points. You can also muffle it if your optimization point is not just thermal efficiency but overall system efficiency. It's really difficult to muffle a turbjoet or turbofan...the exhaust flow is the energy source.