Megggz

I'm having a similar problem with my 2001 Forester, it is squeeling at ~1800 rpm. Doesn't matter what gear or speed I'm at, always 1800 rpm. It seems to stop once the car is warmed up. A mechanic suggested it may be a loose heat sheild (?) so he welded something back on, but the noise is still there. Does anyone know what else it may be? Thanks for your help.