Squieling Noise on Starting

My 1.4 Elegance is making a Squieling / Screeching noise when I start the car it does not do it every time but is happening more often than not. Once started the car runs ok. Except for a couple of times when the car is at temp and staionary the idle drops and hunts a quick rev on the pedal and it goes back to being perfect. It seems to happen in wet or damp weather and it then can be fine again for days. Not effecting fuel, performance or how the engine feels. Any advice would be great as this car is trying to test me and is starting to win.