I had this same noise in my boot (1994 C180 Euro) last year . It sounded like a buzzing sound every 1 or 2 seconds for about 15 minutes from start of every journey. The sound goes away after a while. It comes back after I restart the car for another journey. Very irritating sound for rear passengers. My indie took down the whole dash and found 2 airflow actuators had torn rubber diaphrams which therefore couldn't hold vacuum. These were replaced and the noise stopped except for just 1 buzz when the engine is switched on. The main problem the leak caused was the rearheadrests refused to come down when the button was depressed with the engine running. Not enough vacuum. This year, recently, my centre a/c vents are working off and on and I think there are more problems with the vacuum system. Can anyone help out on this? Most of the time no or very little comes out from the centre vents, sometimes it works properly for a few kilos then stops blowing completely. The side vents work properly all the time.