For the past week I had this mind boggling rattle that drove me nuts! It sounded like a ball bearing that was loose in a metal tray. When I would accelorate it would roll. When I would brake it would roll. Hard corners it would roll. It absolutely sounded like metal on metal. This narrowed down things like stuff in the glove box, junk in the trunk, etc.
I thought the sound eminated from somewhere behind me or below the seats or console so I forced my wife to lay prone in the backseats, her feet hanging out the passenger window listening, while I drove chicanes in the neighborhoods reinacting the noise. She was not happy but carried on just shy of vomiting.
Finally, I removed the seat in my C4S and inserted a small stool in its place. We went out for another test drive. We switched places this time, my wife incredibly agreeing to drive the "bench" seat. With the help of an automotive stethascope I narrowed the rolling metallic noise to a ECU boxed area and a captive frame member below where the seat bolts in.
After returning to the garage and much more experimenting and searching we deduced that the nose came from a closed frame member directly below the ECU box under the seat. I drilled a 1/4 hole in the frame member and used magnets to no avail and finally used sticky grease on a wire and tiny long surgical tweezers to pull a fuse out of the hole I drilled. It seems that this fuse must have, at some point, fallen through the threaded bolt hole that holds the ECU to the floorboard. (on top of this frame member)That's the only explanation I can think of. Or it was put there by some German as a bad joke and finally got dislodged!
<img src="http://www.fototime.com/B710664C3A074CA/standard.jpg" border=0>
The ECU is on the left.
<img src="http://www.fototime.com/4A0182056EB4BA3/standard.jpg" border=0>
It was hard getting the fuse through the tiny hole. I ended up using sticky grease on wire to stand it upright and then surgical tweezers to pull it through.
<img src="http://www.fototime.com/65EB44350D39560/standard.jpg" border=0>
On the plus side I did a full interior clean afterwards. Carpet shampoo (EF), leather clean and dressing (ZAINO), dash dressing, 303, windows, Eagle1, Seals, Gummi Pflege. Spent over 6 hours just on that. Never did it so well. But with the interior gutted I had the opportunity!
Only you, Brad, would do all that to track down an interior rattle. 
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