i think youre hearing the turbo spinning a little..
have you popped the hood while its doing it and tried to narrow down where the sound is coming from?
If it isn't making a boiling noise when you turn the car off, it probably isn't boiling. Boiling coolant doesn't care much if the engine is running or not.
I've noticed something like that too - my guess is the "boiling" sound I is hot oil cookin' in the turbo (which is why I'm installing a turbo timer this weekend).. i mistook it for the rad because it's right up at the front...
