I. Ignition start car . After start key still turned makes screech
What year and how many miles? Screech makes me think the serpentine belt may need to be replaced. A fuller description might help too.
And how long has it been since the fanbelt (or serpentine belt) has been changed?
Any other symptoms?
Can you explain that a little bit? Does the screech happen while you still have the key turned to the Start position? And why, if its started do you still have the key turned to start?
Or have you started the engine and get a screech for some seconds after the engine starts?
Please give the year of the car, the engine type and the transmission, too, so we can assess better.