I haven't had my 335i for long, but there has been this sliding noise that has been driving me crazy when turning, not
accelerating. Everything I had read on here was saying it was coming from the air vents from under the seats. Seemed logical, it sounded like it was a metal abacus each time I turned. I pulled a seat out last night to get the coins out and I was shocked to hear the noise when I lifted the seat out at an angle! The coins were IN the seats!!!
After looking around and yanking on stuff, I realized the coins got in via the knee bolsters! I pulled those out, and pulled the leather and a flap opened up and the coins were right there!
FYI, their are two voids where the coins can go, one directly below this, and one slightly behind that, not visible but I had coins in both and could easily reach my hand back and grab them. What an easy fix, check this first before removing your seats, heck before shoving your hand under there even!!!!