There is virtually no car that is completely quiet and doesn't produce at least a small amount of noise while the engine is running. However, there is quite a difference between today's cars and those from 30-40 years ago. Technology today helps car manufacturers produce some pretty quiet vehicles, with electric and hybrid cars at the top of almost all quietest cars lists. The noise level is usually measured with sophisticated sound level meters to determine the decibel level both inside and outside the car. Besides hybrids and electric cars, diesel-powered cars provide the lowest noise levels due to the fact that diesel engines work at lower revs. The following three cars are considered by most experts to be some of the quietest cars on the market today: Mercedes S350 CDI, Lexus LS 600h L and BMW 335d coupe.
The Mercedes S350 CDI is a diesel-powered luxury car which only produces 63.3 dB. What makes it extra quiet, on top of the diesel engine, is the automatic transmission which can disengage at once when you hit the brakes and the car comes to a full stop. On top of that, it has a very good fuel economy and pretty low emission levels.
The Lexus LS 600h L is a hybrid sedan that runs on 5.0-liter V8 engine combined with two electric motors. Its noise levels are the same as those of the Mercedes S350 CDI, and it's a continuation of the Lexus tradition of making super-quiet cars. When only working on the electric motors, it is perhaps the quietest car in the world, but even when the engine is employed, it's still quieter than most cars.
The third quietest car, the BMW 335d coupe, is slightly worse than the other two cars on this list. It also owes its quietness to the diesel engine, which is a twin-turbo 3.0-liter engine putting out 290 horsepower, and it's not a surprise at all that it fall into this category, considering the fact that most German cars share this characteristic.
So, the conclusion that can be drawn from this is that the quietest cars are mostly luxurious, high-priced vehicles, and that petrol-powered cars definitely fall behind the ones running on diesel engines and the hybrid and electric vehicles.