Thanks.
When I reconnected the battery the following morning the noise disappeared, but I whipped it up to the garage anyway and he couldn't find anything wrong with it. I can hear a bit of a rumbling from the rear nearside wheel too no idea if that's anything to do with it - can't tell if it's from the speaker or the wheel, it's had it's service not long ago though and he reckoned everything was ok. Could have been the alarm, it's a non standard one but it seemed a faff to bother to take apart half the dash when the noise is now stopped. Cheers for the tips, I just hope it stays silent now

Regards Kim