The timing belt will not screech for 20 second then stop. The tension on your PS or ALT A/C belt is probably slack or one of them is worn out. Even if your belts aren't cracked they can still be dried and/or worn out. If you want to know which one it is. try spraying WD-40 on them while they are screeching. When you spray the right one it will stop.
Then you'll know what one is loose or needs replacement.