Sdi 1.9 - rapid noise from under the bonnet

I can't hear anything from inside the car with the radio on, and the car drives fine so I don't know how long this noise has been there, but when I started the car and left it idling while I cleared my windscreen the other day, I heard a rapid firing sort of noise from somewhere under the bonnet. It's quite loud but not quite a steady pattern.

As I was in a rush, I hoped it would be ok and drove to work, and indeed it went fine. I didn't turn the radio on and listened carefully and the noise seemed to stop once I'd got going a little way down the road.

Any ideas?! As I said, no noticeable difference in how it drives, but I don't want to be damaging something.... It makes the noise first thing in the morning and at the end of the day when it's been parked at work. however if I make a few short journeys in a row, it doesn't seem to make the noise.

From looking on line it sounds similar to this?  http://www.youtube.com/watch?v=VeY_3HcC1MI

Edited 1 October, 201411 yr by samfan121