A popular mod for E86 N52 Z4's was removing two foam pieces from a thing called sound generator, which is a pipe running from the intake to the firewall. N52 1 Series have also a similar thing installed, but haven't seen that many people modifying it.
Look up Z4 sound generator on youtube.
Number 10 in this diagram:
http://www.realoem.com/bmw/enUS/show...diagId=13_1165
Edit: 135i also have a sound generator, according to ETK. Dang. Someone be the guinea pig, pls.
https://www.ecstuning.com/b-genuine-...r/13717575658/