Hello,
More seriously, if your car has stock/OEM exhaust untampered and properly operating it must (at least) conform to your country's legislation as it is.
Regarding "properly operating" you should have a
flap (#11) there and it affects the sound:
.
(
They say it works differently in M135i and M140i.)
So, if you wish to play with it,
they say you can use
Bimmerlink.