Since I bought my Boxster I have always noticed a rattle beind the drivers (left) rear, I had it checked out but they never could reproduce the rattle. Seemed like it would happen when I shift gears or got above 60mph. But again not always (60% of the time)
Well this weekend I was cleaning the car and thought I would try to de-snorkle and see what all the fuss was about, and after I got the vent off what do I find inside but four pebbles about the size of your thumbnail!
Removed the pebbles and have not heard the rattle yet after a "nice" ride, and as far as the sound after the de-snorkle I really could not tell any difference at all, so I plan to put the snorkle back in.
Travis 
Found my rattle noise!
Hybrid Mode
