I've heard the same sound coming from my 335is, kinda annoyed me cause i was sitting at a drive thru window & it could be clearly heard reflecting off the building. That was about a week ago and I don't recall hearing it since. My first thought was that it prob is a part of the Emission system cycling on and off.
This is my first BMW so I could be way off.
2011 335is Coupe - S Gray/Red. Prem, Conv & Cold Weather Pkg. DCT, Navi, Harman w/Sat, Ipod & USB, Mats, Tint. MSRP $62,225 - Paid $57,449 OTD