I could be wrong, but it sounds very much like the noise that mine made until I unplugged the wiring connector from the 'active' exhaust tailpipe side & lo & behold no more noise.
Waiting on my dealer confirming I am right & what they are going to do about it.
Last i heard was they need another M135i to compare the two cars side by side.
So it takes seconds to unplug the connector & try it.
Let me know how you get on.
Cheers
Colin
