These sounds can be real hard to localize. I had nearly the same thing in my car when I first got it, and it sounded as if it was coming from the dash by the AC buttons. My tech listened to my description of the noise, and identified it immediatly. He said that small items placed on the dash tended to fall down into the defroster grill openings and work down into the the ducts, and often as far as the under-carpet ductwork, usually on the driver's side. Sure enough, he found a pen in the duct under the front carpet, that rolled around mostly on left hand corners. Removed the pen, and the noise has never come back! Easy enough to get to that I would check there first before moving on to the rest of the ductwork.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E