A Peugeot 407 V6 diesel coupe. Startlingly quiet due to an almost complete absence of wind noise. Cheap 2nd hand as well because they don't sell well. On the launch, we were even warned that the cars are so quiet we may find ourselves going faster than we thought. Jaguar XF 3.0 diesels are pretty good, too, though preferably on 17” or 18” wheels rather than 19” or 20”. ‘Autocar’ still publishes cabin noise levels.