I had the same sound when I first got my 95 850T Wagon. Turns out there was a pencil that had found it's way under the carpet and was rolling around on the metal floor. If you reach your hand under the back of the seat and feel along the carpet, as you get towards the front of the seat, the carpet actually lifts up and you can reach your hand under the carpet and feel the metal floor board- don't recall why it is this way unless it was for the heat for the rear seats. This is how I found the pencil rolling around back and forth as I accelerated or hit the brakes. Get under that seat and cop a feel.

Good Luck!

Keith