[streetpro]

The described decoupling applies to the e92, apparently it can also be applied to the e93.
Lift the rear seats by pulling them up, pull them towards each other, remove the backrest sides, carefully unscrew the decorative leaves on the side upholstery using a plastic screwdriver. Unscrew the 2x torx under the leaves, pull the side upholstery towards the tunnel and disconnect the cable from the belt feed, speaker and lighting. Next is the rubber-plastic decal, held on 3x plastic clips, 2x torx for the sensor and I think 5x hex screw of the metal decal.
Now we will de-grease the sheet metal underrun, which as you can see lacks any damping, as well as the plastic covers (they also sound on tap). This whole acts like an empty drum or a bass enclosure.
We apply alu-butyl hifi anti-vibration material (the best you can afford, no point in doing this whole thing and gluing something with a damping factor of 0.2), over it a rubber carpet to dampen the noise, for sure we can add a few pieces of foam to dampen high frequencies (but it's over-kill).
The next pass equals silence from behind, but it takes away the annoyance that now you can hear the noise from the front... and that's what damping is all about, eliminating one source will show noise elsewhere where it wasn't heard before because it was drowned out by the first.

Enjoy

[streetpro]

PS - now that you know... you may use this quite a large space to smuggle whatever you need to :-)

[Thumbie]

Very nice! So you notice a pretty significant drop in road noise from the rear?

[streetpro]

It's better, but experimentally I found that another very noisy source is the rear seat cushions, or rather the sheet metal under them. So, we have to cover the sheet metal with damping plates and put a noise barrier on top of it (here we have to pay attention to the total thickness of the layers, we have to fit within 1 cm, otherwise you won't put the seat back into the holder).

[streetpro]

I raised the rear seats and damped the sheet metal above the tank - alubutyl (against vibrations) plus a damping layer (against noise). Since I also did the roof, there is a significant difference in the car before/after.

[Thumbie]

Wow, very cool