Compared to my diesel GMC pickup and the diesel Mercedes my parents drove in the 70s and 80s, the X5 35d is very smooth and quiet. Compared to my wife's Lexus RX350, you can definitely hear the X5 more, but then again you can also "feel" much more in the X5 than in the numb and lifeless Lexus. My wife likes overboosted steering, floaty rides, and total isolation, while I like to be reminded that the car is alive!