bhg - noise does seem to come from the pc - it's not stopping/preventing anything from working - eg, if I'm in the middle of writing an email or something on here, then I can carry on.
If you open the Control Panel and select "sound" and then open the "sounds" tab, you can get it to demonstrate all the sounds the software generates eg, close program, low battery. The one you describe sounds like "device disconnect".
1ozzy may well be right. On the other hand, discs are sometimes set to power-off after a period of inactivity as part of a power-saving option - they power-up again automatically as soon as you try to use them but there will be a small time-delay. If you have more than one hard disc on your system that could be happening.
it just seems odd that it has only recently started happening (in the past 5 days or less)
The only things which are plugged into pc are ethernet cable (?) - tried wireless but it was a bit on the slow side. As well as the mouse and speakers.
Am wondering if the mouse is coming loose from it's port?
Now I know it's something relatively safe, my mind is at ease :-D