hi Alan,
we have exactly the same problem.
We told the guy who installed it and he's been round a couple of times but.....suprise suprise it's never made the noise when he's been round. I thought it sounded like a relay was switching on and off really quickly but it's so loud.
It does sound exactly like one of those hand buzzer things.
Have you managed to get it sorted?
Jim W
November 2006