[DBU]

I am not familiar with the G03 (X3 M40i), but I can tell you how it works on the F26 (X4 M40i). Keep in mind we have different motors (B58 vs N55), but I wouldn't be surprised if BMW left this unchanged on the G03.

To check this, take a look at under the car, behind the driver's side rear wheel. You will see the exhaust pipe has a valve that close and open the exhaust flap. You should be able to press firmly on the lever and by doing so open the valve. It is spring loaded and takes a firm press to move.

If you want it to stay open (for better sound), go to the trunk of the car. On the driver's side, you probably have a hard cover that is easily removed to gain access to some electrical modules (radio?)

Now look towards the very rear of the car and you should see a small solenoid attached to the chassis with a rubber mount. It will have two vacuum lines and an electrical connection. To leave the exhaust flap open whenever the motor is running, disconnecty both vacuum hoses from the solonoid and connect the two hoses with a male/male vacuum connector, bypassing the solonoid altogether. I found that a common garden sprinkler connector also works.

Whenever the motor starts, vacuum is sent straight to the valve and it will open - and stay open.

On the X4, engaging sport modes does not automatically open the valve. It seems the valve opens when the solonoid receives a signal from the computer at some point when the motor is asked to produce max, or near max, power.