2007 Subaru Impreza 2.5 with 71000. When the engine is cold it sounds like a bird chirping and get progressively louder then stops after 10-15 seconds
Your serpentine belt is most likely loose or badly worn and in need or replacement.
It is possible that the belt tensioner is going bad.
Or, it is possible that a bearing in either the belt tensioner or the belt idler pulley is going bad.
If a bearing in the tensioner or idler pulley seizes up, you will likely lose the belt and wind up stranded.
A visit to your mechanic is needed at this time.
Belt has been replaced 11,000m ago and tightened about 2 months ago the chirping stops for a couple weeks then came back.