My 135i has 57k miles on it but road noises and other rattles were killing me. I did diagnose most noises to be coming from door panels.
So I used the
bavsound guide to open up the door trims and then peeled off foam to expose inner sides of the door.
I'm an audiophile myself and any noise freaks me out.
Bonus: this DIY also helps with midrange bass
I added 80% coverage on door's inner side. This helps reduce midrange resonances and other road noises that intrude into cabin. This also reduces the vibrations. Added 20% coverage around the speaker wherever i feel plastic was thin to help with better midrange.
I also covered the foam with noise insulation sheets where it had leaks (e.g. around cables)
In case someone is wondering, I used 80mil butyl mats
http://amzn.to/2GAYX3b. They're nearly as good as thicker dynamat at fraction the cost
Took a drive this morning to work, its so much better. Most rattles are gone and road noise is so much less noticeable.
Overall, it easily reduces road noise by at least 6db. However, with other noises gone I notice wind noise even more lol. Have to figure out how to tackle that. If anyone has pointers, that would help.
I did this on my F30 3-series earlier and was equally pleased. It's a must do for everyone.