I also have this problem in my 1994 C180 Euro version and the sounds are the same rythmic buzzing after you start the engine and lasts for about 5 to 10 minutes whether you have the A/C on or not. 2 shops have looked at this but cannot find the vacuum leak and have been only too eager to swop a new A$1200 vacuum pump at my cost without any guarantee that this will solve the problem. Once installed, they won't remove it and replace the old one. Also all the time you are paying for their labour at such exhorbitant rates. Can someone out there pinpoint the exact cause for all of us sufferers? Thanks.