^^That sound is normal. All our 135i's(N54's & N55's) make that noise. It's mostly the valvetrain and somwhat the direct injectors that make that noise. You can also try a different engine oil... that seems to help. At least I noticed a change of "loudness" in my car.
You should also read these threads...
N54 engine ticking, is this a normal noise?
http://www.1addicts.com/forums/showthread.php?t=1187456
Has anyone contacted BMW regarding 29.2 and lag?
http://www.e90post.com/forums/showthread.php?t=147781
^^So many people seem to complain about this non issue... that BMW came up with a SW update to make the engines run much quieter... only problem is this SW update kills throttle "tip in" performance and ultimate Hp.(felt like lost ~20 Hp). Basically BMW detuned our cars with out our permission bc keeping the waste gates open until 3,500 rpms.
After I had this SW update done... I hated it so much I went out and got a German SW flash tune to "fix" it. I am so happy now.
SO basically this sound is normal and you should learn to get use to it. Unless you want BMW to detune your performance car.
Dackel