Finally installed the Berk Exhaust street performance exhaust package:
This is on a 135i N55 engine.
I noticed that there is an annoying hi frequency sound inside the cabin at certain rpms. It hurts the ears.
The sound is similar to when you puncture a tire and air is coming out. whhhhhhsh sound but it quite irritating.
Lowering the windows a little helps.
Other mods include the BMS air intake.