My 2000 Legacy has recently developed this high pitch squeal. It only happens when moving so I guess it's not a belt and doesn't happen in reverse. Also only happens between about 0-35 km/h and goes away after a few hundred metres. I've recorded what it sounds like from inside the car and posted it in the youtube clip below. (pictures are just me messing round when I first bought my Legacy)

Anyone have an idea what might be causing this? 

Cheers!

Automatic or manual transmission? If automatic, does it change or go away when it shifts?

It's a manual transmission.

I hate to sound stupid but it sounds like the brk.pad wear indicators.....or it has a pebble or something like that stuck around the caliper....

I think you could be right, there is a road being re-sealed round the corner so a stone could well have got caught... I should have thought of that!!