When considering how to go about improving fuel efficiency and reducing emissions, we typically think of smaller engines, alternative fuels and electrification. One area that doesn't typically come to mind is sound design and acoustics. While the noises produced by a vehicle don't directly affect fuel efficiency, the effects propagate into areas that do.
Companies like BMW, General Motors and undoubtedly others have been actively pursuing mechanisms of controlling noise at the source in order to create more flexibility for modifying the powertrains. A prime example is the 2010 Chevrolet Equinox with its 2.4-liter inline four-cylinder engine. GM added active noise cancellation so it could allow the engine to spend more time running at lower speeds where it tends to make more of a low booming sound. Without the noise cancellation, the noise would turn off drivers.
Similarly, BMW has been carefully studying the structures of engine blocks and engine bays. Careful application of ribs and tuning of the structural shapes allows these structures to be made lighter without risk of excessive vibration. Similarly, adding insulation materials closer to the engine also reduces the amount needed overall as well as facilitating the use of other types of engines that might be inherently noisier.
All of these factors play into making more efficient vehicles that are appealing to customers. After all, if they don't want to buy the vehicle for reasons that have nothing to do with function, all of the efficiency advantages in the world will go to waste.