m8 I have exactly the same problem on my S2, its a nightmare but I seem to have sourced the rattle, I think.......
I found it to be coming from underneath the turbo, on the stem that runs off the actuator(not 100% sure if thats what the part is but it definetly was coming from there)
it sounds like a very light metal scraping/crackling/vibrating noise which seems to be some kind of fixing with a spring and i think a small nut and washer. get the car up on a ramp or axle stands and start it up.
From underneath the car reach up and grasp the fixing and if this is the cause you should find that the rattle is silent when you have hold of the fixing components.
hope this is of use