I had this issue 2 weeks ago while I was attending a car control clinic. I asked the instructors and they thought I got a rock caught in the car. I tried backing up to clear it and that did not work. The sound comes during idle and at low speeds only. I took it to the dealer and of course they could not find anything wrong with the car and the sound went away while at the shop. They said all exhaust connections were good too. It sounds like something metallic is hitting another metal part in the left rear section. It went away for a while then came back on the other night. I'm still clueless as to the cause.
09 E90 MDCT Silverstone, Black Extended, Prem/Tech/Cold, Carbon Leather, 19's, Enhanced Sound, IPod, PDC, 3M clear bra, 3M tint 35% front, 20% rear, painted reflectors