I've heard this sound a few times too but not for a while now.. Today, returning home from work I heard it again whilst in traffic. I put the window down to hear it coming from the offside rear (driver's side in uk). Most embarrassing! Funny how it disappears again after leaving the car for a while. I'll go out again later but if it is still there and continues in the morning then I'll drive to the dealer and have them listen in...