Either it is
1: worn wheel bearing
2: almost worn out brake pads
on left side front wheel.
When turning right and there is that whistling sound, try to press brake pedal (first make sure it is safe to press brake pedal
) does the whistling sound disappear or will it go louder?
If it disappears, most likely it is worn wheel bearing. If the whistling sound goes louder it most likely is worn out brake pads.
If the sound does not appear always when turning right, the reason is pain to find... if so, it could be the bearing or the brake pads.
You can't find power steering fluid resevoir because your Mini has electric-power-steering.