BLACKSNAKE427

It Sounds Like You Are Some What Mechanicaly Inclined. Just To Rule This Out I Would Like For You To Remove Your Serpentine Belt. Make Sure You Have The Belt Routing Written Down Some Where Before Removing. Remember Some Cars The Belt Can Go Back On In The Wrong Direction Around The Pulleys And Look Good But Can Turn Say Your Waterpump Backwards. But By Doing This You Are Removing From The Equation The Alternator, Waterpump, Etc. Your Saying A Screeching Noise Which Could Be Coming From These Belt Driven Accesories. It Can Be Possible An Accesories Bearings Have Enough Slop In Them That At An Idle It Is Just Enough To Screech. I Hope All This Makes Some Sense. After Removing Belt Start The Mustang And See If Noise Is Still There W/o Overheating The Motor. If Noise Is Still There Then It Has Nothing To Do With The Accesories. If Noise Is Not There Then Start Looking At The Accesories. With The Motor Off And Belt Still Off If It Is The Accesories Making The Noise Try Spinning Each Pulley By Hand See Which One Is Making The Noise. Remember Noise Travels In All Directions And With The Hood Down Noise Takes The Easiest Route To Escape Which Is Down Under The Car. Try This Should Only Take About 20-30 Minutes. Let Me Know What You Find Out. I Am Curious.. I Will Be Back Tommorow Night