[Chris Stewart]

Sometimes when the serpentine belt(fan belt) gets old, it'll develope a shiny glaze on it and slips under the condition you describe. If you know the engine rpm the squeal shows up, take the belt off, start the motor while the car is parked and rev the motor to that previousily noisey rpm and see if it still squeals.....no squeal means the belt is likely bad. Also spin the pulleys to the water pump, power steering, alternator and the idler pulleys for any thing different than a dead smooth spin when you turn them.