Sirion 1.0lt 2006 high pitched engine squeal (only 29,600 miles)

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Hi everyone..
I have a problem with my Sirion SE 1.0 ltr (2006 model), which is not easy to pin point. It is an intermittent loud squeak or squeal coming from the engine somewhere when hot. It sounds like an external noise rather than inside the engine. The noise starts and either goes away for a few seconds and comes back at the same frequency or just gets gradually louder and stays there.
The baffling thing is, that if I turn off the engine at the point of hearing the loud noise and start it up again immediately, it doesn't always start making the noise again! It doesn't seem to be affected (starts or stops) by the automatic engine cooling fan or use of aircon or cabin heaters for that matter. Some days it doesn't make the noise at all, and other days it is very loud and high pitched. The engine has only covered 29,600 miles and is serviced every 7000 miles approx (annual mileage!) and oil level is checked regularly.
I should also point out that it makes the noise especially when parked running idle or when driving at a constant slower speed ie:30mph 4th gear.
Any thoughts, opinions etc would be much appreciated. I thought this would be the best place to find out something.
Thanks.