The title pretty much sums it up.
If I haven't driven the car in a few days, the BMW E46 engine makes a faint chirp for about 30 seconds when I start up the engine. It's more like a rapid chirp, as opposed to a constant squeal, ahe sound goes away when I rev the engine. When I put my ear up close, it sounds like it's coming from near the power steering reservoir, which is located just above the belt. Image here: http://photos.linquist.net/photos/421026382_Js3AM-M.jpg
What could it be?