Our shop 335i has this same problem & has had it since 20k miles.
Could be the injectors opening/closing, valve tap, lifter tick, exhaust leak. Could be quite a few things. Most people don't notice it on their own car because they never have the hood open when the engine is on. I'm guessing the sound is consistent with RPM?
Last edited by Tiago@VRSF; 01-23-2013 at 02:13 PM.