Having often heard a buzzing noise from my MC tdci on engine stop, and having recently turbo problems I did a bit of searching and found what seems to be the cause of the buzzing at least. It seems to be quite common. The turbo boost is controlled by a vacuum modulator driven by PWM from
PCM. The buzzing comes from that valve. Anyone else wondering what this buzzing might be, take a look at this:
www.pierburg-
service.
de/ximages/pg_
si_0076_en_web.pdf