The M135i's exhaust is plenty noisy enough from the outside, but because the cabin is so well sound insulated, you don't hear much. There's something called a sound symposer, this plays the sound of a fruity straight 6 which changes note all through the rev range though the rear speakers.
It is of course fake, but it still sounds nice.
I believe the new GTI has it, along with a load of other cars to come. sh*tty EU ruling on cabin db and this seems to be the only way around it.