Noise reduction is quite simple. Take a steel wire sponge which is used to clean kitchen pots (no idea what is the perfect translation) and stuff it into the exhaust pipe. Did work well on my M3W when I had to reduce noise due to an official test
Maybe the drawing can explain my adaptation for the test. The distance holder was made of two little steel plates

[Linked Image]

Hannes
once: Green M3W; 2013
now: Red 4/4 Sport; 2011
and some practical cars for use in real life