I've actually made a recording of the buzzing which, as I mentioned, is totally unpredictable and varies in length from 2 seconds to over a minute, but I can't see how to post any attached file on this site.
It's really quite loud, louder than an electric clock alarm for example, and is really quite irritating. My wife and I suspect it might be the solenoid or an electrical relay(?) but my knowledge on these matters is very limited.
Steve
January 2008