Hmm, interesting. Actually I pointed out that switching off the ignition stopped the sound both times. When I stopped, left the engine running and listened for the sound it just continued once I started moving again. Did you get any additional info?

Yes, it's very loud when the window is down, has yours gone away by this morning? It has happened twice in the last month and just goes away.

Still looking at changing the water pump, thermostat and belts next week as I am now 6 years and 90k miles on the original - I am just paranoid the water pump is going to fail and leave me stranded.