Only does it for the amount of time it takes for the car to start, so literally a quick shirp sound then stops once it fires up.
I had this problem mate and three garages later I took it to a specialist and they said its a glaze on the belt mixed with what Danny just said.
mine also does it, but as said not worried, alternator charging the battery!
cold weather gets to batteries. The alternator is having to work a bit harder straight after startup
It doesn't do it after start up! It's does it in the time between turning the key and the car starting.