I too get a buzzing noise when I first switch on the ignition. It stops after 15 secs or so (and it doesn't always do it anyway) and it does sound for all the world like it might be a little electric priming thing that is ensuring the fuel pump is primed. But this is only conjecture - I have absolutely no grounds for stating that it IS that. Cant recall, but think I raised it on BF a while back and got some suggestions back, most of them centering on glow plug energising I think, though I'm at a loss to understand why they should make a noise like wot I have been hearing.
It occurs to me. In te age of recording mp3 player, internet etc. there's no reason why one shouldnt try recording sounds and put them up for listening in to and comment on BF. Must add that to the long list of things I ought to try

Mike
