Well, I have experienced this twice during my 6 years with minidisc recording live with microphone...
First time was with my sony mzr-30 (bought 98) and second time was yesterday with my sony mzr-909.
I have found that in both cases it was the remote who was generating the hi-frequent buzzing sound. If I unplug my remote, the recordings get as silent (when there is silence) as they possibly can be.
I have searched minidisc.org for any other users experience of this, found nothing - quite weird - there must be other users who have discovered this too?
I would love to have som kind of info on w h y this buzz occurs, my advice is however to keep the remote away from your unit while recording.